[feat](trx-rs): add FT4 decoder support

Reuse the existing ft8_lib C library (FTX_PROTOCOL_FT4) and FT8
decoder infrastructure to add FT4 decoding across the full stack.

Changes:
- trx-ft8: add protocol param to ft8_decoder_create; add Ft8Decoder::new_ft4()
- trx-core: DecodedMessage::Ft4 variant, AUDIO_MSG_FT4_DECODE (0x14),
  ft4_decode_enabled/ft4_decode_reset_seq state, SetFt4DecodeEnabled/
  ResetFt4Decoder commands, protocol mapping
- trx-server: DecoderHistories::ft4, run_ft4_decoder (7.5s slots via
  now*2/15), run_background_ft4_decoder, history push/replay, decoder
  task spawn
- trx-frontend-http: ft4_history in FrontendRuntimeContext,
  toggle/clear endpoints, /ft4.js route, bookmark/scheduler/background
  decode support, DecodeHistoryPayload ft4 field
- web: ft4.js plugin (7.5s period timer, reuses FT8 CSS/map infra),
  FT4 subtab in index.html, app.js dispatch (onServerFt4/Batch,
  restoreFt4History), decode-history-worker HISTORY_GROUP_KEYS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/decoders/trx-ft8/src/ft8_wrapper.c

@@ -108,7 +108,7 @@ typedef struct
     float freq_hz;
 } ft8_decode_result_t;
 
-ft8_decoder_t* ft8_decoder_create(int sample_rate, float f_min, float f_max, int time_osr, int freq_osr)
+ft8_decoder_t* ft8_decoder_create(int sample_rate, float f_min, float f_max, int time_osr, int freq_osr, int protocol)
 {
     ft8_decoder_t* dec = (ft8_decoder_t*)calloc(1, sizeof(ft8_decoder_t));
     if (!dec)
@@ -120,7 +120,7 @@ ft8_decoder_t* ft8_decoder_create(int sample_rate, float f_min, float f_max, int
     dec->cfg.sample_rate = sample_rate;
     dec->cfg.time_osr = time_osr;
     dec->cfg.freq_osr = freq_osr;
-    dec->cfg.protocol = FTX_PROTOCOL_FT8;
+    dec->cfg.protocol = (protocol == 0) ? FTX_PROTOCOL_FT4 : FTX_PROTOCOL_FT8;
 
     hashtable_init();
     monitor_init(&dec->mon, &dec->cfg);

src/decoders/trx-ft8/src/lib.rs

@@ -37,6 +37,7 @@ extern "C" {
         f_max: c_float,
         time_osr: c_int,
         freq_osr: c_int,
+        protocol: c_int,
     ) -> *mut c_void;
     fn ft8_decoder_free(dec: *mut c_void);
     fn ft8_decoder_block_size(dec: *const c_void) -> c_int;
@@ -69,6 +70,7 @@ impl Ft8Decoder {
                 F_MAX_HZ,
                 TIME_OSR as c_int,
                 FREQ_OSR as c_int,
+                1, // FTX_PROTOCOL_FT8
             );
             let inner = NonNull::new(ptr).ok_or_else(|| "ft8_decoder_create failed".to_string())?;
             let block_size = ft8_decoder_block_size(inner.as_ptr()) as usize;
@@ -84,6 +86,30 @@ impl Ft8Decoder {
         }
     }
 
+    pub fn new_ft4(sample_rate: u32) -> Result<Self, String> {
+        unsafe {
+            let ptr = ft8_decoder_create(
+                sample_rate as c_int,
+                F_MIN_HZ,
+                F_MAX_HZ,
+                TIME_OSR as c_int,
+                FREQ_OSR as c_int,
+                0, // FTX_PROTOCOL_FT4
+            );
+            let inner = NonNull::new(ptr).ok_or_else(|| "ft8_decoder_create failed".to_string())?;
+            let block_size = ft8_decoder_block_size(inner.as_ptr()) as usize;
+            if block_size == 0 {
+                ft8_decoder_free(inner.as_ptr());
+                return Err("invalid FT4 block size".to_string());
+            }
+            Ok(Self {
+                inner,
+                block_size,
+                sample_rate,
+            })
+        }
+    }
+
     pub fn block_size(&self) -> usize {
         self.block_size
     }

src/trx-client/src/audio_client.rs

@@ -24,7 +24,7 @@ use uuid::Uuid;
 use trx_core::audio::{
     parse_vchan_audio_frame, parse_vchan_uuid_msg, read_audio_msg, write_audio_msg,
     write_vchan_uuid_msg, AudioStreamInfo, AUDIO_MSG_AIS_DECODE, AUDIO_MSG_APRS_DECODE,
-    AUDIO_MSG_CW_DECODE, AUDIO_MSG_FT8_DECODE, AUDIO_MSG_HF_APRS_DECODE,
+    AUDIO_MSG_CW_DECODE, AUDIO_MSG_FT4_DECODE, AUDIO_MSG_FT8_DECODE, AUDIO_MSG_HF_APRS_DECODE,
     AUDIO_MSG_HISTORY_COMPRESSED, AUDIO_MSG_RX_FRAME, AUDIO_MSG_RX_FRAME_CH,
     AUDIO_MSG_STREAM_INFO, AUDIO_MSG_TX_FRAME, AUDIO_MSG_VCHAN_ALLOCATED, AUDIO_MSG_VCHAN_BW,
     AUDIO_MSG_VCHAN_DESTROYED, AUDIO_MSG_VCHAN_FREQ, AUDIO_MSG_VCHAN_MODE, AUDIO_MSG_VCHAN_REMOVE,
@@ -287,6 +287,7 @@ async fn handle_audio_connection(
                     | AUDIO_MSG_HF_APRS_DECODE
                     | AUDIO_MSG_CW_DECODE
                     | AUDIO_MSG_FT8_DECODE
+                    | AUDIO_MSG_FT4_DECODE
                     | AUDIO_MSG_WSPR_DECODE,
                     payload,
                 )) => {

src/trx-client/src/main.rs

@@ -363,6 +363,9 @@ async fn async_init() -> DynResult<AppState> {
                             history.push_back((now, message));
                         }
                     }
+                    DecodedMessage::Ft4(_) => {
+                        // FT4 history is managed by the frontend HTTP audio collector
+                    }
                     DecodedMessage::Wspr(message) => {
                         if let Ok(mut history) = wspr_history.lock() {
                             history.push_back((now, message));

src/trx-client/src/remote_client.rs

@@ -776,6 +776,7 @@ mod tests {
             hf_aprs_decode_enabled: false,
             cw_decode_enabled: false,
             ft8_decode_enabled: false,
+            ft4_decode_enabled: false,
             wspr_decode_enabled: false,
             cw_auto: true,
             cw_wpm: 15,

src/trx-client/trx-frontend/src/lib.rs

@@ -202,6 +202,8 @@ pub struct FrontendRuntimeContext {
     pub cw_history: Arc<Mutex<VecDeque<(Instant, CwEvent)>>>,
     /// FT8 decode history (timestamp, message)
     pub ft8_history: Arc<Mutex<VecDeque<(Instant, Ft8Message)>>>,
+    /// FT4 decode history (timestamp, message)
+    pub ft4_history: Arc<Mutex<VecDeque<(Instant, Ft8Message)>>>,
     /// WSPR decode history (timestamp, message)
     pub wspr_history: Arc<Mutex<VecDeque<(Instant, WsprMessage)>>>,
     /// Authentication tokens for HTTP-JSON frontend
@@ -283,6 +285,7 @@ impl FrontendRuntimeContext {
             hf_aprs_history: Arc::new(Mutex::new(VecDeque::new())),
             cw_history: Arc::new(Mutex::new(VecDeque::new())),
             ft8_history: Arc::new(Mutex::new(VecDeque::new())),
+            ft4_history: Arc::new(Mutex::new(VecDeque::new())),
             wspr_history: Arc::new(Mutex::new(VecDeque::new())),
             auth_tokens: HashSet::new(),
             sse_clients: Arc::new(AtomicUsize::new(0)),

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -214,11 +214,13 @@ function applyAuthRestrictions() {
       "ais-clear-btn",
       "vdes-clear-btn",
       "ft8-decode-toggle-btn",
+      "ft4-decode-toggle-btn",
       "wspr-decode-toggle-btn",
       "hf-aprs-decode-toggle-btn",
       "cw-auto",
       "aprs-clear-btn",
       "ft8-clear-btn",
+      "ft4-clear-btn",
       "wspr-clear-btn",
       "cw-clear-btn"
     ];
@@ -2847,6 +2849,13 @@ function render(update) {
     ft8ToggleBtn.style.borderColor = ft8On ? "#00d17f" : "";
     ft8ToggleBtn.style.color = ft8On ? "#00d17f" : "";
   }
+  const ft4ToggleBtn = document.getElementById("ft4-decode-toggle-btn");
+  if (ft4ToggleBtn) {
+    const ft4On = !!update.ft4_decode_enabled;
+    ft4ToggleBtn.textContent = ft4On ? "Disable FT4" : "Enable FT4";
+    ft4ToggleBtn.style.borderColor = ft4On ? "#00d17f" : "";
+    ft4ToggleBtn.style.color = ft4On ? "#00d17f" : "";
+  }
   const wsprToggleBtn = document.getElementById("wspr-decode-toggle-btn");
   if (wsprToggleBtn) {
     const wsprOn = !!update.wspr_decode_enabled;
@@ -7312,6 +7321,7 @@ function updateDecodeStatus(text) {
   const aprs = document.getElementById("aprs-status");
   const cw = document.getElementById("cw-status");
   const ft8 = document.getElementById("ft8-status");
+  const ft4 = document.getElementById("ft4-status");
   setModeBoundDecodeStatus(ais, ["AIS"], "Select AIS mode to decode", text);
   const vdesText = text === "Connected, listening for packets" ? "Connected, listening for bursts" : text;
   setModeBoundDecodeStatus(vdes, ["VDES"], "Select VDES mode to decode", vdesText);
@@ -7319,6 +7329,7 @@ function updateDecodeStatus(text) {
   const cwText = text === "Connected, listening for packets" ? "Connected, listening for CW" : text;
   setModeBoundDecodeStatus(cw, ["CW", "CWR"], "Select CW mode to decode", cwText);
   if (ft8 && ft8.textContent !== "Receiving") ft8.textContent = text;
+  if (ft4 && ft4.textContent !== "Receiving") ft4.textContent = text;
 }
 function dispatchDecodeMessage(msg) {
   if (msg.type === "ais" && window.onServerAis) window.onServerAis(msg);
@@ -7327,6 +7338,7 @@ function dispatchDecodeMessage(msg) {
   if (msg.type === "hf_aprs" && window.onServerHfAprs) window.onServerHfAprs(msg);
   if (msg.type === "cw" && window.onServerCw) window.onServerCw(msg);
   if (msg.type === "ft8" && window.onServerFt8) window.onServerFt8(msg);
+  if (msg.type === "ft4" && window.onServerFt4) window.onServerFt4(msg);
   if (msg.type === "wspr" && window.onServerWspr) window.onServerWspr(msg);
 }
 
@@ -7355,6 +7367,10 @@ function dispatchDecodeBatch(batch) {
       window.onServerFt8Batch(batch);
       return;
     }
+    if (type === "ft4" && window.onServerFt4Batch) {
+      window.onServerFt4Batch(batch);
+      return;
+    }
     if (type === "wspr" && window.onServerWsprBatch) {
       window.onServerWsprBatch(batch);
       return;
@@ -7425,6 +7441,10 @@ function restoreDecodeHistoryGroup(kind, messages) {
     window.restoreFt8History(messages);
     return;
   }
+  if (kind === "ft4" && window.restoreFt4History) {
+    window.restoreFt4History(messages);
+    return;
+  }
   if (kind === "wspr" && window.restoreWsprHistory) {
     window.restoreWsprHistory(messages);
     return;
@@ -7441,6 +7461,7 @@ function connectDecode() {
   if (window.resetAprsHistoryView) window.resetAprsHistoryView();
   if (window.resetCwHistoryView) window.resetCwHistoryView();
   if (window.resetFt8HistoryView) window.resetFt8HistoryView();
+  if (window.resetFt4HistoryView) window.resetFt4HistoryView();
   if (window.resetWsprHistoryView) window.resetWsprHistoryView();
 
   // Buffer live messages until history fetch settles so history always appears
@@ -7512,7 +7533,7 @@ function connectDecode() {
 
   function totalDecodeHistoryMessages(groups) {
     if (!groups || typeof groups !== "object") return 0;
-    return ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "wspr"]
+    return ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "wspr"]
       .reduce((sum, key) => sum + (Array.isArray(groups[key]) ? groups[key].length : 0), 0);
   }
 
@@ -7523,7 +7544,7 @@ function connectDecode() {
       setDecodeHistoryReplayActive(true);
       updateHistoryReplayOverlay();
     }
-    for (const kind of ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "wspr"]) {
+    for (const kind of ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "wspr"]) {
       const messages = groups && Array.isArray(groups[kind]) ? groups[kind] : [];
       if (messages.length === 0) continue;
       for (let index = 0; index < messages.length; index += DECODE_HISTORY_WORKER_GROUP_LIMIT) {

src/trx-client/trx-frontend/trx-frontend-http/assets/web/decode-history-worker.js

@@ -1,5 +1,5 @@
 const textDecoder = typeof TextDecoder === "function" ? new TextDecoder() : null;
-const HISTORY_GROUP_KEYS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "wspr"];
+const HISTORY_GROUP_KEYS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "wspr"];
 
 function decodeCborUint(view, bytes, state, additional) {
   const offset = state.offset;

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -374,6 +374,7 @@
                 <label class="bm-decoder-check"><input type="checkbox" id="bm-dec-aprs" value="aprs" /> APRS</label>
                 <label class="bm-decoder-check"><input type="checkbox" id="bm-dec-ais" value="ais" /> AIS</label>
                 <label class="bm-decoder-check"><input type="checkbox" id="bm-dec-ft8" value="ft8" /> FT8</label>
+                <label class="bm-decoder-check"><input type="checkbox" id="bm-dec-ft4" value="ft4" /> FT4</label>
                 <label class="bm-decoder-check"><input type="checkbox" id="bm-dec-wspr" value="wspr" /> WSPR</label>
                 <label class="bm-decoder-check"><input type="checkbox" id="bm-dec-hf-aprs" value="hf-aprs" /> HF APRS</label>
               </div>
@@ -425,6 +426,7 @@
         <button class="sub-tab" data-subtab="hf-aprs">HF APRS</button>
         <button class="sub-tab" data-subtab="cw">CW</button>
         <button class="sub-tab" data-subtab="ft8">FT8</button>
+        <button class="sub-tab" data-subtab="ft4">FT4</button>
         <button class="sub-tab" data-subtab="wspr">WSPR</button>
         <button class="sub-tab" data-subtab="rds">RDS</button>
       </div>
@@ -634,6 +636,24 @@
         </div>
         <div id="ft8-messages"></div>
       </div>
+      <div id="subtab-ft4" class="sub-tab-panel" style="display:none;">
+        <div class="ft8-controls">
+          <button id="ft4-decode-toggle-btn" type="button">Enable FT4</button>
+          <button id="ft4-pause-btn" type="button">Pause</button>
+          <button id="ft4-clear-btn" type="button">Clear</button>
+          <input id="ft4-filter" class="ft8-filter" type="text" placeholder="Filter (e.g. CQ, DL4)" />
+          <small id="ft4-status" style="color:var(--text-muted);">Waiting for server decode</small>
+          <small id="ft4-period" style="color:var(--text-muted);">Next slot --s</small>
+        </div>
+        <div class="ft8-header">
+          <span class="ft8-time">Time</span>
+          <span class="ft8-snr">SNR</span>
+          <span class="ft8-dt">DT</span>
+          <span class="ft8-freq">RF</span>
+          <span class="ft8-msg">Message</span>
+        </div>
+        <div id="ft4-messages"></div>
+      </div>
       <div id="subtab-wspr" class="sub-tab-panel" style="display:none;">
         <div class="ft8-controls">
           <button id="wspr-decode-toggle-btn" type="button">Enable WSPR</button>
@@ -910,6 +930,7 @@
   <script src="/aprs.js"></script>
   <script src="/hf-aprs.js"></script>
   <script src="/ft8.js"></script>
+  <script src="/ft4.js"></script>
   <script src="/wspr.js"></script>
   <script src="/cw.js"></script>
   <script src="/bookmarks.js"></script>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/ft4.js

@@ -0,0 +1,191 @@
+// --- FT4 Decoder Plugin (server-side decode) ---
+// SPDX-FileCopyrightText: 2026 Stanislaw Grams <stanislawgrams@gmail.com>
+// SPDX-License-Identifier: BSD-2-Clause
+
+function ft8RenderMessage(message) {
+  if (typeof renderFt8Message === "function") return renderFt8Message(message);
+  if (typeof ft8EscapeHtml === "function") return ft8EscapeHtml(message);
+  return message;
+}
+
+const ft4Status = document.getElementById("ft4-status");
+const ft4PeriodEl = document.getElementById("ft4-period");
+const ft4MessagesEl = document.getElementById("ft4-messages");
+const ft4FilterInput = document.getElementById("ft4-filter");
+const ft4PauseBtn = document.getElementById("ft4-pause-btn");
+const FT4_PERIOD_MS = 7500;
+let ft4FilterText = "";
+let ft4MessageHistory = [];
+let ft4Paused = false;
+let ft4BufferedWhilePaused = 0;
+
+function currentFt4HistoryRetentionMs() {
+  return typeof window.getDecodeHistoryRetentionMs === "function"
+    ? window.getDecodeHistoryRetentionMs()
+    : 24 * 60 * 60 * 1000;
+}
+
+function pruneFt4MessageHistory() {
+  const cutoffMs = Date.now() - currentFt4HistoryRetentionMs();
+  ft4MessageHistory = ft4MessageHistory.filter((msg) => Number(msg?._tsMs ?? msg?.ts_ms) >= cutoffMs);
+}
+
+function scheduleFt4Ui(key, job) {
+  if (typeof window.trxScheduleUiFrameJob === "function") {
+    window.trxScheduleUiFrameJob(key, job);
+    return;
+  }
+  job();
+}
+
+function scheduleFt4HistoryRender() { scheduleFt4Ui("ft4-history", () => renderFt4History()); }
+
+function normalizeFt4DisplayFreqHz(freqHz) {
+  const rawHz = Number(freqHz);
+  if (!Number.isFinite(rawHz)) return null;
+  const baseHz = Number.isFinite(window.ft8BaseHz) ? Number(window.ft8BaseHz) : null;
+  if (Number.isFinite(baseHz) && baseHz > 0 && rawHz >= 0 && rawHz < 100000) {
+    return baseHz + rawHz;
+  }
+  return rawHz;
+}
+
+function updateFt4PeriodTimer() {
+  if (!ft4PeriodEl) return;
+  const nowMs = Date.now();
+  const remaining = (FT4_PERIOD_MS - nowMs % FT4_PERIOD_MS) / 1000;
+  ft4PeriodEl.textContent = `Next slot ${remaining.toFixed(1)}s`;
+}
+
+updateFt4PeriodTimer();
+setInterval(updateFt4PeriodTimer, 250);
+
+function renderFt4Row(msg) {
+  const row = document.createElement("div");
+  row.className = "ft8-row";
+  const rawMessage = (msg.message || "").toString();
+  row.dataset.message = rawMessage.toUpperCase();
+  row.dataset.decoder = "ft4";
+  row.dataset.storedFreqHz = Number.isFinite(msg.freq_hz) ? String(msg.freq_hz) : "";
+  const snr = Number.isFinite(msg.snr_db) ? msg.snr_db.toFixed(1) : "--";
+  const dt = Number.isFinite(msg.dt_s) ? msg.dt_s.toFixed(2) : "--";
+  const displayFreqHz = normalizeFt4DisplayFreqHz(msg.freq_hz);
+  const freq = Number.isFinite(displayFreqHz) ? displayFreqHz.toFixed(0) : "--";
+  const renderedMessage = ft8RenderMessage(rawMessage);
+  const tsMs = msg._tsMs ?? msg.ts_ms;
+  const timeStr = tsMs ? new Date(tsMs).toLocaleTimeString([], { hour: "2-digit", minute: "2-digit", second: "2-digit" }) : "--:--:--";
+  row.innerHTML = `<span class="ft8-time">${timeStr}</span><span class="ft8-snr">${snr}</span><span class="ft8-dt">${dt}</span><span class="ft8-freq">${freq}</span><span class="ft8-msg">${renderedMessage}</span>`;
+  return row;
+}
+
+function updateFt4PauseUi() {
+  if (!ft4PauseBtn) return;
+  ft4PauseBtn.textContent = ft4Paused ? "Resume" : "Pause";
+  ft4PauseBtn.classList.toggle("active", ft4Paused);
+}
+
+function renderFt4History() {
+  pruneFt4MessageHistory();
+  if (!ft4MessagesEl || ft4Paused) { updateFt4PauseUi(); return; }
+  const filter = ft4FilterText;
+  const fragment = document.createDocumentFragment();
+  for (let i = 0; i < ft4MessageHistory.length; i++) {
+    const msg = ft4MessageHistory[i];
+    if (filter && !(msg.message || "").toString().toUpperCase().includes(filter)) continue;
+    fragment.appendChild(renderFt4Row(msg));
+  }
+  ft4MessagesEl.replaceChildren(fragment);
+  updateFt4PauseUi();
+}
+
+function addFt4Message(msg) {
+  msg._tsMs = Number.isFinite(msg?.ts_ms) ? Number(msg.ts_ms) : Date.now();
+  ft4MessageHistory.unshift(msg);
+  pruneFt4MessageHistory();
+  if (ft4Paused) { ft4BufferedWhilePaused += 1; updateFt4PauseUi(); return; }
+  scheduleFt4HistoryRender();
+}
+
+function normalizeServerFt4Message(msg) {
+  const raw = (msg.message || "").toString();
+  const locatorDetails = typeof ft8ExtractLocatorDetails === "function" ? ft8ExtractLocatorDetails(raw) : [];
+  const grids = locatorDetails.length > 0
+    ? locatorDetails.map((d) => d.grid)
+    : (typeof ft8ExtractAllGrids === "function" ? ft8ExtractAllGrids(raw) : []);
+  const station = typeof ft8ExtractLikelyCallsign === "function" ? ft8ExtractLikelyCallsign(raw) : null;
+  const rfHz = normalizeFt4DisplayFreqHz(msg.freq_hz);
+  return {
+    raw, grids, station, rfHz, locatorDetails,
+    history: {
+      receiver: window.getDecodeRigMeta ? window.getDecodeRigMeta() : null,
+      ts_ms: msg.ts_ms, snr_db: msg.snr_db, dt_s: msg.dt_s,
+      freq_hz: Number.isFinite(rfHz) ? rfHz : msg.freq_hz,
+      message: msg.message,
+    },
+  };
+}
+
+window.onServerFt4Batch = function(messages) {
+  if (!Array.isArray(messages) || messages.length === 0) return;
+  if (ft4Status) ft4Status.textContent = ft4Paused ? "Paused" : "Receiving";
+  const normalized = [];
+  for (const msg of messages) {
+    const next = normalizeServerFt4Message(msg);
+    if (next.grids.length > 0 && window.ft8MapAddLocator) {
+      window.ft8MapAddLocator(next.raw, next.grids, "ft8", next.station, { ...msg, freq_hz: next.rfHz, locator_details: next.locatorDetails });
+    }
+    next.history._tsMs = Number.isFinite(next.history?.ts_ms) ? Number(next.history.ts_ms) : Date.now();
+    normalized.push(next.history);
+  }
+  normalized.reverse();
+  ft4MessageHistory = normalized.concat(ft4MessageHistory);
+  pruneFt4MessageHistory();
+  if (ft4Paused) { ft4BufferedWhilePaused += messages.length; updateFt4PauseUi(); return; }
+  scheduleFt4HistoryRender();
+};
+
+window.restoreFt4History = function(messages) { window.onServerFt4Batch(messages); };
+window.pruneFt4HistoryView = function() { pruneFt4MessageHistory(); renderFt4History(); };
+
+window.resetFt4HistoryView = function() {
+  if (ft4MessagesEl) ft4MessagesEl.innerHTML = "";
+  ft4MessageHistory = [];
+  ft4BufferedWhilePaused = 0;
+  renderFt4History();
+};
+
+if (ft4FilterInput) {
+  ft4FilterInput.addEventListener("input", () => {
+    ft4FilterText = ft4FilterInput.value.trim().toUpperCase();
+    renderFt4History();
+  });
+}
+
+if (ft4PauseBtn) {
+  ft4PauseBtn.addEventListener("click", () => {
+    ft4Paused = !ft4Paused;
+    if (!ft4Paused) { ft4BufferedWhilePaused = 0; renderFt4History(); } else { updateFt4PauseUi(); }
+  });
+}
+
+document.getElementById("ft4-decode-toggle-btn")?.addEventListener("click", async () => {
+  try { await postPath("/toggle_ft4_decode"); } catch (e) { console.error("FT4 toggle failed", e); }
+});
+
+document.getElementById("ft4-clear-btn")?.addEventListener("click", async () => {
+  try {
+    await postPath("/clear_ft4_decode");
+    window.resetFt4HistoryView();
+  } catch (e) { console.error("FT4 clear failed", e); }
+});
+
+window.onServerFt4 = function(msg) {
+  if (ft4Status) ft4Status.textContent = ft4Paused ? "Paused" : "Receiving";
+  const next = normalizeServerFt4Message(msg);
+  if (next.grids.length > 0 && window.ft8MapAddLocator) {
+    window.ft8MapAddLocator(next.raw, next.grids, "ft8", next.station, { ...msg, freq_hz: next.rfHz, locator_details: next.locatorDetails });
+  }
+  addFt4Message(next.history);
+};
+
+updateFt4PauseUi();

src/trx-client/trx-frontend/trx-frontend-http/src/api.rs

@@ -450,6 +450,7 @@ struct DecodeHistoryPayload {
     hf_aprs: Vec<trx_core::decode::AprsPacket>,
     cw: Vec<trx_core::decode::CwEvent>,
     ft8: Vec<trx_core::decode::Ft8Message>,
+    ft4: Vec<trx_core::decode::Ft8Message>,
     wspr: Vec<trx_core::decode::WsprMessage>,
 }
 
@@ -461,6 +462,7 @@ impl DecodeHistoryPayload {
             + self.hf_aprs.len()
             + self.cw.len()
             + self.ft8.len()
+            + self.ft4.len()
             + self.wspr.len()
     }
 }
@@ -474,6 +476,7 @@ fn collect_decode_history(context: &FrontendRuntimeContext) -> DecodeHistoryPayl
         hf_aprs: crate::server::audio::snapshot_hf_aprs_history(context),
         cw: crate::server::audio::snapshot_cw_history(context),
         ft8: crate::server::audio::snapshot_ft8_history(context),
+        ft4: crate::server::audio::snapshot_ft4_history(context),
         wspr: crate::server::audio::snapshot_wspr_history(context),
     }
 }
@@ -1002,6 +1005,15 @@ pub async fn toggle_ft8_decode(
     send_command(&rig_tx, RigCommand::SetFt8DecodeEnabled(!enabled)).await
 }
 
+#[post("/toggle_ft4_decode")]
+pub async fn toggle_ft4_decode(
+    state: web::Data<watch::Receiver<RigState>>,
+    rig_tx: web::Data<mpsc::Sender<RigRequest>>,
+) -> Result<HttpResponse, Error> {
+    let enabled = state.get_ref().borrow().ft4_decode_enabled;
+    send_command(&rig_tx, RigCommand::SetFt4DecodeEnabled(!enabled)).await
+}
+
 #[post("/toggle_wspr_decode")]
 pub async fn toggle_wspr_decode(
     state: web::Data<watch::Receiver<RigState>>,
@@ -1020,6 +1032,15 @@ pub async fn clear_ft8_decode(
     send_command(&rig_tx, RigCommand::ResetFt8Decoder).await
 }
 
+#[post("/clear_ft4_decode")]
+pub async fn clear_ft4_decode(
+    context: web::Data<Arc<FrontendRuntimeContext>>,
+    rig_tx: web::Data<mpsc::Sender<RigRequest>>,
+) -> Result<HttpResponse, Error> {
+    crate::server::audio::clear_ft4_history(context.get_ref());
+    send_command(&rig_tx, RigCommand::ResetFt4Decoder).await
+}
+
 #[post("/clear_wspr_decode")]
 pub async fn clear_wspr_decode(
     context: web::Data<Arc<FrontendRuntimeContext>>,
@@ -1502,6 +1523,7 @@ pub fn configure(cfg: &mut web::ServiceConfig) {
         .service(set_cw_wpm)
         .service(set_cw_tone)
         .service(toggle_ft8_decode)
+        .service(toggle_ft4_decode)
         .service(toggle_wspr_decode)
         .service(clear_ais_decode)
         .service(clear_vdes_decode)
@@ -1509,6 +1531,7 @@ pub fn configure(cfg: &mut web::ServiceConfig) {
         .service(clear_hf_aprs_decode)
         .service(clear_cw_decode)
         .service(clear_ft8_decode)
+        .service(clear_ft4_decode)
         .service(clear_wspr_decode)
         .service(select_rig)
         // Bookmark CRUD
@@ -1542,6 +1565,7 @@ pub fn configure(cfg: &mut web::ServiceConfig) {
         .service(aprs_js)
         .service(hf_aprs_js)
         .service(ft8_js)
+        .service(ft4_js)
         .service(wspr_js)
         .service(cw_js)
         .service(bookmarks_js)
@@ -1715,6 +1739,13 @@ async fn ft8_js() -> impl Responder {
         .body(status::FT8_JS)
 }
 
+#[get("/ft4.js")]
+async fn ft4_js() -> impl Responder {
+    HttpResponse::Ok()
+        .insert_header((header::CONTENT_TYPE, "application/javascript; charset=utf-8"))
+        .body(status::FT4_JS)
+}
+
 #[get("/wspr.js")]
 async fn wspr_js() -> impl Responder {
     HttpResponse::Ok()
@@ -1848,10 +1879,11 @@ async fn send_command_to_rig(
 
 fn bookmark_decoder_state(
     bookmark: &crate::server::bookmarks::Bookmark,
-) -> (bool, bool, bool, bool) {
+) -> (bool, bool, bool, bool, bool) {
     let mut want_aprs = bookmark.mode.trim().eq_ignore_ascii_case("PKT");
     let mut want_hf_aprs = false;
     let mut want_ft8 = false;
+    let mut want_ft4 = false;
     let mut want_wspr = false;
 
     for decoder in bookmark
@@ -1863,12 +1895,13 @@ fn bookmark_decoder_state(
             "aprs" => want_aprs = true,
             "hf-aprs" => want_hf_aprs = true,
             "ft8" => want_ft8 = true,
+            "ft4" => want_ft4 = true,
             "wspr" => want_wspr = true,
             _ => {}
         }
     }
 
-    (want_aprs, want_hf_aprs, want_ft8, want_wspr)
+    (want_aprs, want_hf_aprs, want_ft8, want_ft4, want_wspr)
 }
 
 fn bookmark_decoder_kinds(bookmark: &crate::server::bookmarks::Bookmark) -> Vec<String> {
@@ -1880,7 +1913,7 @@ fn bookmark_decoder_kinds(bookmark: &crate::server::bookmarks::Bookmark) -> Vec<
     {
         if matches!(
             decoder.as_str(),
-            "aprs" | "ais" | "ft8" | "wspr" | "hf-aprs"
+            "aprs" | "ais" | "ft8" | "ft4" | "wspr" | "hf-aprs"
         ) && !out.iter().any(|existing| existing == &decoder)
         {
             out.push(decoder);
@@ -1935,11 +1968,12 @@ async fn apply_selected_channel(
     let Some(bookmark) = bookmark_store.get(bookmark_id) else {
         return Ok(());
     };
-    let (want_aprs, want_hf_aprs, want_ft8, want_wspr) = bookmark_decoder_state(&bookmark);
+    let (want_aprs, want_hf_aprs, want_ft8, want_ft4, want_wspr) = bookmark_decoder_state(&bookmark);
     let desired = [
         RigCommand::SetAprsDecodeEnabled(want_aprs),
         RigCommand::SetHfAprsDecodeEnabled(want_hf_aprs),
         RigCommand::SetFt8DecodeEnabled(want_ft8),
+        RigCommand::SetFt4DecodeEnabled(want_ft4),
         RigCommand::SetWsprDecodeEnabled(want_wspr),
     ];
     for cmd in desired {
@@ -1987,6 +2021,7 @@ async fn wait_for_view(mut rx: watch::Receiver<RigState>) -> Result<RigSnapshot,
         cw_wpm: state.cw_wpm,
         cw_tone_hz: state.cw_tone_hz,
         ft8_decode_enabled: state.ft8_decode_enabled,
+        ft4_decode_enabled: state.ft4_decode_enabled,
         wspr_decode_enabled: state.wspr_decode_enabled,
         filter: state.filter.clone(),
         spectrum: None,

src/trx-client/trx-frontend/trx-frontend-http/src/audio.rs

@@ -147,6 +147,16 @@ fn prune_ft8_history(context: &FrontendRuntimeContext, history: &mut VecDeque<(I
     }
 }
 
+fn prune_ft4_history(context: &FrontendRuntimeContext, history: &mut VecDeque<(Instant, Ft8Message)>) {
+    let cutoff = decode_history_cutoff(context);
+    while let Some((ts, _)) = history.front() {
+        if *ts >= cutoff {
+            break;
+        }
+        history.pop_front();
+    }
+}
+
 fn prune_wspr_history(
     context: &FrontendRuntimeContext,
     history: &mut VecDeque<(Instant, WsprMessage)>,
@@ -202,6 +212,15 @@ fn record_ft8(context: &FrontendRuntimeContext, msg: Ft8Message) {
     prune_ft8_history(context, &mut history);
 }
 
+fn record_ft4(context: &FrontendRuntimeContext, msg: Ft8Message) {
+    let mut history = context
+        .ft4_history
+        .lock()
+        .expect("ft4 history mutex poisoned");
+    history.push_back((Instant::now(), msg));
+    prune_ft4_history(context, &mut history);
+}
+
 fn record_wspr(context: &FrontendRuntimeContext, msg: WsprMessage) {
     let mut history = context
         .wspr_history
@@ -280,6 +299,15 @@ pub fn snapshot_ft8_history(context: &FrontendRuntimeContext) -> Vec<Ft8Message>
     history.iter().map(|(_, msg)| msg.clone()).collect()
 }
 
+pub fn snapshot_ft4_history(context: &FrontendRuntimeContext) -> Vec<Ft8Message> {
+    let mut history = context
+        .ft4_history
+        .lock()
+        .expect("ft4 history mutex poisoned");
+    prune_ft4_history(context, &mut history);
+    history.iter().map(|(_, msg)| msg.clone()).collect()
+}
+
 pub fn snapshot_wspr_history(context: &FrontendRuntimeContext) -> Vec<WsprMessage> {
     let mut history = context
         .wspr_history
@@ -337,6 +365,14 @@ pub fn clear_ft8_history(context: &FrontendRuntimeContext) {
     history.clear();
 }
 
+pub fn clear_ft4_history(context: &FrontendRuntimeContext) {
+    let mut history = context
+        .ft4_history
+        .lock()
+        .expect("ft4 history mutex poisoned");
+    history.clear();
+}
+
 pub fn clear_wspr_history(context: &FrontendRuntimeContext) {
     let mut history = context
         .wspr_history
@@ -374,6 +410,7 @@ pub fn start_decode_history_collector(context: Arc<FrontendRuntimeContext>) {
                     DecodedMessage::HfAprs(pkt) => record_hf_aprs(&context, pkt),
                     DecodedMessage::Cw(evt) => record_cw(&context, evt),
                     DecodedMessage::Ft8(msg) => record_ft8(&context, msg),
+                    DecodedMessage::Ft4(msg) => record_ft4(&context, msg),
                     DecodedMessage::Wspr(msg) => record_wspr(&context, msg),
                 },
                 Err(broadcast::error::RecvError::Lagged(_)) => continue,

src/trx-client/trx-frontend/trx-frontend-http/src/background_decode.rs

@@ -21,7 +21,7 @@ use crate::server::bookmarks::{Bookmark, BookmarkStore};
 use crate::server::scheduler::{SchedulerStatusMap, SharedSchedulerControlManager};
 use crate::server::vchan::{ClientChannel, ClientChannelManager};
 
-const SUPPORTED_DECODER_KINDS: &[&str] = &["aprs", "ais", "ft8", "wspr", "hf-aprs"];
+const SUPPORTED_DECODER_KINDS: &[&str] = &["aprs", "ais", "ft8", "ft4", "wspr", "hf-aprs"];
 const CHANNEL_KIND_NAME: &str = "VirtualBackgroundDecodeChannel";
 const VISIBLE_CHANNEL_KIND_NAME: &str = "VirtualChannel";
 

src/trx-client/trx-frontend/trx-frontend-http/src/scheduler.rs

@@ -672,6 +672,7 @@ async fn apply_scheduler_decoders(
     let mut want_aprs = bookmark.mode.trim().eq_ignore_ascii_case("PKT");
     let mut want_hf_aprs = false;
     let mut want_ft8 = false;
+    let mut want_ft4 = false;
     let mut want_wspr = false;
 
     let mut update_from = |bm: &crate::server::bookmarks::Bookmark| {
@@ -680,6 +681,7 @@ async fn apply_scheduler_decoders(
                 "aprs" => want_aprs = true,
                 "hf-aprs" => want_hf_aprs = true,
                 "ft8" => want_ft8 = true,
+                "ft4" => want_ft4 = true,
                 "wspr" => want_wspr = true,
                 _ => {}
             }
@@ -695,6 +697,7 @@ async fn apply_scheduler_decoders(
         ("APRS", RigCommand::SetAprsDecodeEnabled(want_aprs)),
         ("HF APRS", RigCommand::SetHfAprsDecodeEnabled(want_hf_aprs)),
         ("FT8", RigCommand::SetFt8DecodeEnabled(want_ft8)),
+        ("FT4", RigCommand::SetFt4DecodeEnabled(want_ft4)),
         ("WSPR", RigCommand::SetWsprDecodeEnabled(want_wspr)),
     ];
 

src/trx-client/trx-frontend/trx-frontend-http/src/status.rs

@@ -19,6 +19,7 @@ pub const VDES_JS: &str = include_str!("../assets/web/plugins/vdes.js");
 pub const APRS_JS: &str = include_str!("../assets/web/plugins/aprs.js");
 pub const HF_APRS_JS: &str = include_str!("../assets/web/plugins/hf-aprs.js");
 pub const FT8_JS: &str = include_str!("../assets/web/plugins/ft8.js");
+pub const FT4_JS: &str = include_str!("../assets/web/plugins/ft4.js");
 pub const WSPR_JS: &str = include_str!("../assets/web/plugins/wspr.js");
 pub const CW_JS: &str = include_str!("../assets/web/plugins/cw.js");
 pub const BOOKMARKS_JS: &str = include_str!("../assets/web/plugins/bookmarks.js");

src/trx-core/src/audio.rs

@@ -62,6 +62,8 @@ pub const AUDIO_MSG_VCHAN_DESTROYED: u8 = 0x12;
 /// Client → server: update the audio filter bandwidth of an existing virtual channel.
 /// Payload: JSON `{"uuid": "<uuid>", "bandwidth_hz": <u32>}`.
 pub const AUDIO_MSG_VCHAN_BW: u8 = 0x13;
+/// Server → client: FT4 decoded message (JSON `DecodedMessage::Ft4`).
+pub const AUDIO_MSG_FT4_DECODE: u8 = 0x14;
 
 /// Maximum payload size for normal messages (1 MB).
 const MAX_PAYLOAD_SIZE: u32 = 1_048_576;

src/trx-core/src/decode.rs

@@ -22,6 +22,8 @@ pub enum DecodedMessage {
     Cw(CwEvent),
     #[serde(rename = "ft8")]
     Ft8(Ft8Message),
+    #[serde(rename = "ft4")]
+    Ft4(Ft8Message),
     #[serde(rename = "wspr")]
     Wspr(WsprMessage),
 }

src/trx-core/src/rig/command.rs

@@ -28,11 +28,13 @@ pub enum RigCommand {
     SetCwWpm(u32),
     SetCwToneHz(u32),
     SetFt8DecodeEnabled(bool),
+    SetFt4DecodeEnabled(bool),
     SetWsprDecodeEnabled(bool),
     ResetAprsDecoder,
     ResetHfAprsDecoder,
     ResetCwDecoder,
     ResetFt8Decoder,
+    ResetFt4Decoder,
     ResetWsprDecoder,
     SetBandwidth(u32),
     SetFirTaps(u32),

src/trx-core/src/rig/controller/handlers.rs

@@ -509,12 +509,14 @@ pub fn command_from_rig_command(cmd: RigCommand) -> Box<dyn RigCommandHandler> {
         | RigCommand::SetCwWpm(_)
         | RigCommand::SetCwToneHz(_)
         | RigCommand::SetFt8DecodeEnabled(_)
+        | RigCommand::SetFt4DecodeEnabled(_)
         | RigCommand::SetWsprDecodeEnabled(_)
         | RigCommand::SetHfAprsDecodeEnabled(_)
         | RigCommand::ResetHfAprsDecoder
         | RigCommand::ResetAprsDecoder
         | RigCommand::ResetCwDecoder
         | RigCommand::ResetFt8Decoder
+        | RigCommand::ResetFt4Decoder
         | RigCommand::ResetWsprDecoder
         | RigCommand::SetBandwidth(_)
         | RigCommand::SetFirTaps(_)

src/trx-core/src/rig/state.rs

@@ -38,6 +38,8 @@ pub struct RigState {
     #[serde(default)]
     pub ft8_decode_enabled: bool,
     #[serde(default)]
+    pub ft4_decode_enabled: bool,
+    #[serde(default)]
     pub wspr_decode_enabled: bool,
     #[serde(default)]
     pub cw_auto: bool,
@@ -66,6 +68,8 @@ pub struct RigState {
     #[serde(default, skip_serializing)]
     pub ft8_decode_reset_seq: u64,
     #[serde(default, skip_serializing)]
+    pub ft4_decode_reset_seq: u64,
+    #[serde(default, skip_serializing)]
     pub wspr_decode_reset_seq: u64,
 }
 
@@ -143,6 +147,7 @@ impl RigState {
             hf_aprs_decode_enabled: false,
             cw_decode_enabled: true,
             ft8_decode_enabled: false,
+            ft4_decode_enabled: false,
             wspr_decode_enabled: false,
             cw_auto: true,
             cw_wpm: 15,
@@ -154,6 +159,7 @@ impl RigState {
             hf_aprs_decode_reset_seq: 0,
             cw_decode_reset_seq: 0,
             ft8_decode_reset_seq: 0,
+            ft4_decode_reset_seq: 0,
             wspr_decode_reset_seq: 0,
         }
     }
@@ -210,6 +216,7 @@ impl RigState {
             cw_wpm: snapshot.cw_wpm,
             cw_tone_hz: snapshot.cw_tone_hz,
             ft8_decode_enabled: snapshot.ft8_decode_enabled,
+            ft4_decode_enabled: snapshot.ft4_decode_enabled,
             wspr_decode_enabled: snapshot.wspr_decode_enabled,
             filter: snapshot.filter,
             spectrum: None, // spectrum flows through /api/spectrum, not persistent state
@@ -218,6 +225,7 @@ impl RigState {
             hf_aprs_decode_reset_seq: 0,
             cw_decode_reset_seq: 0,
             ft8_decode_reset_seq: 0,
+            ft4_decode_reset_seq: 0,
             wspr_decode_reset_seq: 0,
         }
     }
@@ -252,6 +260,7 @@ impl RigState {
             cw_wpm: self.cw_wpm,
             cw_tone_hz: self.cw_tone_hz,
             ft8_decode_enabled: self.ft8_decode_enabled,
+            ft4_decode_enabled: self.ft4_decode_enabled,
             wspr_decode_enabled: self.wspr_decode_enabled,
             filter: self.filter.clone(),
             spectrum: self.spectrum.clone(),
@@ -416,6 +425,8 @@ pub struct RigSnapshot {
     #[serde(default)]
     pub ft8_decode_enabled: bool,
     #[serde(default)]
+    pub ft4_decode_enabled: bool,
+    #[serde(default)]
     pub wspr_decode_enabled: bool,
     #[serde(default)]
     pub cw_auto: bool,

src/trx-protocol/src/mapping.rs

@@ -42,6 +42,7 @@ pub fn client_command_to_rig(cmd: ClientCommand) -> RigCommand {
         ClientCommand::SetCwWpm { wpm } => RigCommand::SetCwWpm(wpm),
         ClientCommand::SetCwToneHz { tone_hz } => RigCommand::SetCwToneHz(tone_hz),
         ClientCommand::SetFt8DecodeEnabled { enabled } => RigCommand::SetFt8DecodeEnabled(enabled),
+        ClientCommand::SetFt4DecodeEnabled { enabled } => RigCommand::SetFt4DecodeEnabled(enabled),
         ClientCommand::SetWsprDecodeEnabled { enabled } => {
             RigCommand::SetWsprDecodeEnabled(enabled)
         }
@@ -49,6 +50,7 @@ pub fn client_command_to_rig(cmd: ClientCommand) -> RigCommand {
         ClientCommand::ResetHfAprsDecoder => RigCommand::ResetHfAprsDecoder,
         ClientCommand::ResetCwDecoder => RigCommand::ResetCwDecoder,
         ClientCommand::ResetFt8Decoder => RigCommand::ResetFt8Decoder,
+        ClientCommand::ResetFt4Decoder => RigCommand::ResetFt4Decoder,
         ClientCommand::ResetWsprDecoder => RigCommand::ResetWsprDecoder,
         ClientCommand::SetBandwidth { bandwidth_hz } => RigCommand::SetBandwidth(bandwidth_hz),
         ClientCommand::SetFirTaps { taps } => RigCommand::SetFirTaps(taps),
@@ -100,6 +102,7 @@ pub fn rig_command_to_client(cmd: RigCommand) -> ClientCommand {
         RigCommand::SetCwWpm(wpm) => ClientCommand::SetCwWpm { wpm },
         RigCommand::SetCwToneHz(tone_hz) => ClientCommand::SetCwToneHz { tone_hz },
         RigCommand::SetFt8DecodeEnabled(enabled) => ClientCommand::SetFt8DecodeEnabled { enabled },
+        RigCommand::SetFt4DecodeEnabled(enabled) => ClientCommand::SetFt4DecodeEnabled { enabled },
         RigCommand::SetWsprDecodeEnabled(enabled) => {
             ClientCommand::SetWsprDecodeEnabled { enabled }
         }
@@ -107,6 +110,7 @@ pub fn rig_command_to_client(cmd: RigCommand) -> ClientCommand {
         RigCommand::ResetHfAprsDecoder => ClientCommand::ResetHfAprsDecoder,
         RigCommand::ResetCwDecoder => ClientCommand::ResetCwDecoder,
         RigCommand::ResetFt8Decoder => ClientCommand::ResetFt8Decoder,
+        RigCommand::ResetFt4Decoder => ClientCommand::ResetFt4Decoder,
         RigCommand::ResetWsprDecoder => ClientCommand::ResetWsprDecoder,
         RigCommand::SetBandwidth(bandwidth_hz) => ClientCommand::SetBandwidth { bandwidth_hz },
         RigCommand::SetFirTaps(taps) => ClientCommand::SetFirTaps { taps },

src/trx-protocol/src/types.rs

@@ -33,11 +33,13 @@ pub enum ClientCommand {
     SetCwWpm { wpm: u32 },
     SetCwToneHz { tone_hz: u32 },
     SetFt8DecodeEnabled { enabled: bool },
+    SetFt4DecodeEnabled { enabled: bool },
     SetWsprDecodeEnabled { enabled: bool },
     ResetAprsDecoder,
     ResetHfAprsDecoder,
     ResetCwDecoder,
     ResetFt8Decoder,
+    ResetFt4Decoder,
     ResetWsprDecoder,
     SetBandwidth { bandwidth_hz: u32 },
     SetFirTaps { taps: u32 },

src/trx-server/src/audio.rs

@@ -24,9 +24,9 @@ use trx_aprs::AprsDecoder;
 use trx_core::audio::{
     parse_vchan_uuid_msg, read_audio_msg, write_audio_msg, write_vchan_audio_frame,
     write_vchan_uuid_msg, AudioStreamInfo,
-    AUDIO_MSG_AIS_DECODE, AUDIO_MSG_APRS_DECODE, AUDIO_MSG_CW_DECODE, AUDIO_MSG_FT8_DECODE,
+    AUDIO_MSG_AIS_DECODE, AUDIO_MSG_APRS_DECODE, AUDIO_MSG_CW_DECODE, AUDIO_MSG_FT4_DECODE,
-    AUDIO_MSG_HF_APRS_DECODE, AUDIO_MSG_HISTORY_COMPRESSED, AUDIO_MSG_RX_FRAME,
+    AUDIO_MSG_FT8_DECODE, AUDIO_MSG_HF_APRS_DECODE, AUDIO_MSG_HISTORY_COMPRESSED,
-    AUDIO_MSG_STREAM_INFO, AUDIO_MSG_TX_FRAME, AUDIO_MSG_VCHAN_ALLOCATED,
+    AUDIO_MSG_RX_FRAME, AUDIO_MSG_STREAM_INFO, AUDIO_MSG_TX_FRAME, AUDIO_MSG_VCHAN_ALLOCATED,
     AUDIO_MSG_VCHAN_BW, AUDIO_MSG_VCHAN_DESTROYED, AUDIO_MSG_VCHAN_FREQ, AUDIO_MSG_VCHAN_MODE,
     AUDIO_MSG_VCHAN_REMOVE,
     AUDIO_MSG_VCHAN_SUB, AUDIO_MSG_VCHAN_UNSUB, AUDIO_MSG_VDES_DECODE, AUDIO_MSG_WSPR_DECODE,
@@ -157,6 +157,7 @@ pub struct DecoderHistories {
     pub hf_aprs: Mutex<VecDeque<(Instant, AprsPacket)>>,
     pub cw: Mutex<VecDeque<(Instant, CwEvent)>>,
     pub ft8: Mutex<VecDeque<(Instant, Ft8Message)>>,
+    pub ft4: Mutex<VecDeque<(Instant, Ft8Message)>>,
     pub wspr: Mutex<VecDeque<(Instant, WsprMessage)>>,
 }
 
@@ -169,6 +170,7 @@ impl DecoderHistories {
             hf_aprs: Mutex::new(VecDeque::new()),
             cw: Mutex::new(VecDeque::new()),
             ft8: Mutex::new(VecDeque::new()),
+            ft4: Mutex::new(VecDeque::new()),
             wspr: Mutex::new(VecDeque::new()),
         })
     }
@@ -363,6 +365,35 @@ impl DecoderHistories {
         self.ft8.lock().expect("ft8 history mutex poisoned").clear();
     }
 
+    // --- FT4 ---
+
+    fn prune_ft4(history: &mut VecDeque<(Instant, Ft8Message)>) {
+        let cutoff = Instant::now() - FT8_HISTORY_RETENTION;
+        while let Some((ts, _)) = history.front() {
+            if *ts < cutoff {
+                history.pop_front();
+            } else {
+                break;
+            }
+        }
+    }
+
+    pub fn record_ft4_message(&self, msg: Ft8Message) {
+        let mut h = self.ft4.lock().expect("ft4 history mutex poisoned");
+        h.push_back((Instant::now(), msg));
+        Self::prune_ft4(&mut h);
+    }
+
+    pub fn snapshot_ft4_history(&self) -> Vec<Ft8Message> {
+        let mut h = self.ft4.lock().expect("ft4 history mutex poisoned");
+        Self::prune_ft4(&mut h);
+        h.iter().map(|(_, msg)| msg.clone()).collect()
+    }
+
+    pub fn clear_ft4_history(&self) {
+        self.ft4.lock().expect("ft4 history mutex poisoned").clear();
+    }
+
     // --- WSPR ---
 
     fn prune_wspr(history: &mut VecDeque<(Instant, WsprMessage)>) {
@@ -404,8 +435,9 @@ impl DecoderHistories {
         let hf_aprs = self.hf_aprs.lock().map(|h| h.len()).unwrap_or(0);
         let cw = self.cw.lock().map(|h| h.len()).unwrap_or(0);
         let ft8 = self.ft8.lock().map(|h| h.len()).unwrap_or(0);
+        let ft4 = self.ft4.lock().map(|h| h.len()).unwrap_or(0);
         let wspr = self.wspr.lock().map(|h| h.len()).unwrap_or(0);
-        ais + vdes + aprs + hf_aprs + cw + ft8 + wspr
+        ais + vdes + aprs + hf_aprs + cw + ft8 + ft4 + wspr
     }
 }
 
@@ -1646,6 +1678,148 @@ pub async fn run_ft8_decoder(
     }
 }
 
+/// Run the FT4 decoder task. Mirrors FT8 but uses FT4 protocol (7.5s slots).
+pub async fn run_ft4_decoder(
+    sample_rate: u32,
+    channels: u16,
+    mut pcm_rx: broadcast::Receiver<Vec<f32>>,
+    mut state_rx: watch::Receiver<RigState>,
+    decode_tx: broadcast::Sender<DecodedMessage>,
+    histories: Arc<DecoderHistories>,
+) {
+    info!("FT4 decoder started ({}Hz, {} ch)", sample_rate, channels);
+    let mut decoder = match Ft8Decoder::new_ft4(FT8_SAMPLE_RATE) {
+        Ok(decoder) => decoder,
+        Err(err) => {
+            warn!("FT4 decoder init failed: {}", err);
+            return;
+        }
+    };
+    let mut last_reset_seq: u64 = 0;
+    let mut active = state_rx.borrow().ft4_decode_enabled
+        && matches!(state_rx.borrow().status.mode, RigMode::DIG | RigMode::USB);
+    let mut ft4_buf: Vec<f32> = Vec::new();
+    let mut last_slot: i64 = -1;
+
+    loop {
+        if !active {
+            match state_rx.changed().await {
+                Ok(()) => {
+                    let state = state_rx.borrow();
+                    active = state.ft4_decode_enabled
+                        && matches!(state.status.mode, RigMode::DIG | RigMode::USB);
+                    if active {
+                        pcm_rx = pcm_rx.resubscribe();
+                    }
+                    if state.ft4_decode_reset_seq != last_reset_seq {
+                        last_reset_seq = state.ft4_decode_reset_seq;
+                        decoder.reset();
+                        ft4_buf.clear();
+                    }
+                    last_slot = -1;
+                }
+                Err(_) => break,
+            }
+            continue;
+        }
+
+        tokio::select! {
+            recv = pcm_rx.recv() => {
+                match recv {
+                    Ok(frame) => {
+                        let now = match std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH) {
+                            Ok(dur) => dur.as_secs() as i64,
+                            Err(_) => 0,
+                        };
+                        // FT4 slot period is 7.5s; use now * 2 / 15 for integer slot index
+                        let slot = now * 2 / 15;
+                        if slot != last_slot {
+                            last_slot = slot;
+                            decoder.reset();
+                            ft4_buf.clear();
+                        }
+
+                        let reset_seq = {
+                            let state = state_rx.borrow();
+                            state.ft4_decode_reset_seq
+                        };
+                        if reset_seq != last_reset_seq {
+                            last_reset_seq = reset_seq;
+                            decoder.reset();
+                            ft4_buf.clear();
+                        }
+
+                        let mut mono = downmix_mono(frame, channels);
+                        apply_decode_audio_gate(&mut mono);
+                        let Some(resampled) = resample_to_12k(&mono, sample_rate) else {
+                            warn!("FT4 decoder: unsupported sample rate {}", sample_rate);
+                            break;
+                        };
+                        ft4_buf.extend_from_slice(&resampled);
+
+                        while ft4_buf.len() >= decoder.block_size() {
+                            let block: Vec<f32> = ft4_buf.drain(..decoder.block_size()).collect();
+                            let results = tokio::task::block_in_place(|| {
+                                decoder.process_block(&block);
+                                decoder.decode_if_ready(100)
+                            });
+                            if !results.is_empty() {
+                                for res in results {
+                                    let ts_ms = match std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH) {
+                                        Ok(dur) => dur.as_millis() as i64,
+                                        Err(_) => 0,
+                                    };
+                                    let base_freq_hz = state_rx.borrow().status.freq.hz as f64;
+                                    let abs_freq_hz = base_freq_hz + res.freq_hz as f64;
+                                    let msg = Ft8Message {
+                                        ts_ms,
+                                        snr_db: res.snr_db,
+                                        dt_s: res.dt_s,
+                                        freq_hz: if abs_freq_hz.is_finite() && abs_freq_hz > 0.0 {
+                                            abs_freq_hz as f32
+                                        } else {
+                                            res.freq_hz
+                                        },
+                                        message: res.text,
+                                    };
+                                    histories.record_ft4_message(msg.clone());
+                                    let _ = decode_tx.send(DecodedMessage::Ft4(msg));
+                                }
+                            }
+                        }
+                    }
+                    Err(broadcast::error::RecvError::Lagged(n)) => {
+                        warn!("FT4 decoder: dropped {} PCM frames", n);
+                    }
+                    Err(broadcast::error::RecvError::Closed) => break,
+                }
+            }
+            changed = state_rx.changed() => {
+                match changed {
+                    Ok(()) => {
+                        let state = state_rx.borrow();
+                        active = state.ft4_decode_enabled
+                            && matches!(state.status.mode, RigMode::DIG | RigMode::USB);
+                        if state.ft4_decode_reset_seq != last_reset_seq {
+                            last_reset_seq = state.ft4_decode_reset_seq;
+                            decoder.reset();
+                            ft4_buf.clear();
+                        }
+                        if !active {
+                            decoder.reset();
+                            ft4_buf.clear();
+                            last_slot = -1;
+                        } else {
+                            pcm_rx = pcm_rx.resubscribe();
+                        }
+                    }
+                    Err(_) => break,
+                }
+            }
+        }
+    }
+}
+
 /// Run the WSPR decoder task. Mirrors FT8 lifecycle/slot behavior.
 ///
 /// Note: decoding engine integration is intentionally staged; this task already
@@ -1997,6 +2171,85 @@ async fn run_background_ft8_decoder(
     }
 }
 
+async fn run_background_ft4_decoder(
+    sample_rate: u32,
+    channels: u16,
+    mut pcm_rx: broadcast::Receiver<Vec<f32>>,
+    base_freq_hz: u64,
+    decode_tx: broadcast::Sender<DecodedMessage>,
+) {
+    info!(
+        "Background FT4 decoder started ({}Hz, {} ch @ {} Hz)",
+        sample_rate, channels, base_freq_hz
+    );
+    let mut decoder = match Ft8Decoder::new_ft4(FT8_SAMPLE_RATE) {
+        Ok(decoder) => decoder,
+        Err(err) => {
+            warn!("Background FT4 decoder init failed: {}", err);
+            return;
+        }
+    };
+    let mut ft4_buf: Vec<f32> = Vec::new();
+    let mut last_slot: i64 = -1;
+
+    loop {
+        match pcm_rx.recv().await {
+            Ok(frame) => {
+                let now = match std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH)
+                {
+                    Ok(dur) => dur.as_secs() as i64,
+                    Err(_) => 0,
+                };
+                // FT4 slot period is 7.5s; use now * 2 / 15 for integer slot index
+                let slot = now * 2 / 15;
+                if slot != last_slot {
+                    last_slot = slot;
+                    decoder.reset();
+                    ft4_buf.clear();
+                }
+
+                let mut mono = downmix_mono(frame, channels);
+                apply_decode_audio_gate(&mut mono);
+                let Some(resampled) = resample_to_12k(&mono, sample_rate) else {
+                    warn!(
+                        "Background FT4 decoder: unsupported sample rate {}",
+                        sample_rate
+                    );
+                    break;
+                };
+                ft4_buf.extend_from_slice(&resampled);
+
+                while ft4_buf.len() >= decoder.block_size() {
+                    let block: Vec<f32> = ft4_buf.drain(..decoder.block_size()).collect();
+                    let results = tokio::task::block_in_place(|| {
+                        decoder.process_block(&block);
+                        decoder.decode_if_ready(100)
+                    });
+                    for res in results {
+                        let abs_freq_hz = base_freq_hz as f64 + res.freq_hz as f64;
+                        let msg = Ft8Message {
+                            ts_ms: current_timestamp_ms(),
+                            snr_db: res.snr_db,
+                            dt_s: res.dt_s,
+                            freq_hz: if abs_freq_hz.is_finite() && abs_freq_hz > 0.0 {
+                                abs_freq_hz as f32
+                            } else {
+                                res.freq_hz
+                            },
+                            message: res.text,
+                        };
+                        let _ = decode_tx.send(DecodedMessage::Ft4(msg));
+                    }
+                }
+            }
+            Err(broadcast::error::RecvError::Lagged(n)) => {
+                warn!("Background FT4 decoder: dropped {} PCM frames", n);
+            }
+            Err(broadcast::error::RecvError::Closed) => break,
+        }
+    }
+}
+
 async fn run_background_wspr_decoder(
     sample_rate: u32,
     channels: u16,
@@ -2198,6 +2451,7 @@ async fn handle_audio_client(
         push_history!(histories.snapshot_aprs_history(), DecodedMessage::Aprs, AUDIO_MSG_APRS_DECODE);
         push_history!(histories.snapshot_hf_aprs_history(), DecodedMessage::HfAprs, AUDIO_MSG_HF_APRS_DECODE);
         push_history!(histories.snapshot_ft8_history(), DecodedMessage::Ft8, AUDIO_MSG_FT8_DECODE);
+        push_history!(histories.snapshot_ft4_history(), DecodedMessage::Ft4, AUDIO_MSG_FT4_DECODE);
         push_history!(histories.snapshot_wspr_history(), DecodedMessage::Wspr, AUDIO_MSG_WSPR_DECODE);
         push_history!(histories.snapshot_cw_history(), DecodedMessage::Cw, AUDIO_MSG_CW_DECODE);
 
@@ -2283,6 +2537,7 @@ async fn handle_audio_client(
                                 DecodedMessage::HfAprs(_) => AUDIO_MSG_HF_APRS_DECODE,
                                 DecodedMessage::Cw(_) => AUDIO_MSG_CW_DECODE,
                                 DecodedMessage::Ft8(_) => AUDIO_MSG_FT8_DECODE,
+                                DecodedMessage::Ft4(_) => AUDIO_MSG_FT4_DECODE,
                                 DecodedMessage::Wspr(_) => AUDIO_MSG_WSPR_DECODE,
                             };
                             if let Ok(json) = serde_json::to_vec(&msg) {
@@ -2308,6 +2563,7 @@ async fn handle_audio_client(
                                 DecodedMessage::HfAprs(_) => AUDIO_MSG_HF_APRS_DECODE,
                                 DecodedMessage::Cw(_) => AUDIO_MSG_CW_DECODE,
                                 DecodedMessage::Ft8(_) => AUDIO_MSG_FT8_DECODE,
+                                DecodedMessage::Ft4(_) => AUDIO_MSG_FT4_DECODE,
                                 DecodedMessage::Wspr(_) => AUDIO_MSG_WSPR_DECODE,
                             };
                             if let Ok(json) = serde_json::to_vec(&msg) {
@@ -2438,6 +2694,16 @@ async fn handle_audio_client(
                                             )
                                             .await;
                                         }),
+                                        "ft4" => tokio::spawn(async move {
+                                            run_background_ft4_decoder(
+                                                sr,
+                                                ch_count,
+                                                task_rx,
+                                                base_freq_hz,
+                                                decode_tx,
+                                            )
+                                            .await;
+                                        }),
                                         "wspr" => tokio::spawn(async move {
                                             run_background_wspr_decoder(
                                                 sr,

src/trx-server/src/main.rs

@@ -726,6 +726,21 @@ fn spawn_rig_audio_stack(
             }
         }));
 
+        // Spawn FT4 decoder task
+        let ft4_pcm_rx = pcm_tx.subscribe();
+        let ft4_state_rx = state_rx.clone();
+        let ft4_decode_tx = decode_tx.clone();
+        let ft4_sr = rig_cfg.audio.sample_rate;
+        let ft4_ch = rig_cfg.audio.channels;
+        let ft4_shutdown_rx = shutdown_rx.clone();
+        let ft4_histories = histories.clone();
+        handles.push(tokio::spawn(async move {
+            tokio::select! {
+                _ = audio::run_ft4_decoder(ft4_sr, ft4_ch as u16, ft4_pcm_rx, ft4_state_rx, ft4_decode_tx, ft4_histories) => {}
+                _ = wait_for_shutdown(ft4_shutdown_rx) => {}
+            }
+        }));
+
         // Spawn WSPR decoder task
         let wspr_pcm_rx = pcm_tx.subscribe();
         let wspr_state_rx = state_rx.clone();

src/trx-server/src/rig_task.rs

@@ -470,6 +470,12 @@ async fn process_command(
             let _ = ctx.state_tx.send(ctx.state.clone());
             return snapshot_from(ctx.state);
         }
+        RigCommand::SetFt4DecodeEnabled(en) => {
+            ctx.state.ft4_decode_enabled = en;
+            info!("FT4 decode {}", if en { "enabled" } else { "disabled" });
+            let _ = ctx.state_tx.send(ctx.state.clone());
+            return snapshot_from(ctx.state);
+        }
         RigCommand::SetWsprDecodeEnabled(en) => {
             ctx.state.wspr_decode_enabled = en;
             info!("WSPR decode {}", if en { "enabled" } else { "disabled" });
@@ -505,6 +511,12 @@ async fn process_command(
             let _ = ctx.state_tx.send(ctx.state.clone());
             return snapshot_from(ctx.state);
         }
+        RigCommand::ResetFt4Decoder => {
+            ctx.histories.clear_ft4_history();
+            ctx.state.ft4_decode_reset_seq += 1;
+            let _ = ctx.state_tx.send(ctx.state.clone());
+            return snapshot_from(ctx.state);
+        }
         RigCommand::ResetWsprDecoder => {
             ctx.histories.clear_wspr_history();
             ctx.state.wspr_decode_reset_seq += 1;