diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 917e9a5..a7cfc24 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -422,8 +422,18 @@ multi-modal payloads (text inline, audio/photo/data via `iroh-blobs`). Receiving
             A `GossipPresence` source would emit `MeshEvent::Present`/`Discovered`,
             retiring the SSE presence bus. Confirmed: gossip compiles to WASM, so
             the web client uses the same discovery unchanged (§6.3).
-      - [ ] Milestone 4 — wrap as `IrohTransport` behind the `Transport`/
-            `MeshEvent` contract; A/B against RTC+LAN on latency & connect rate.
+      - [~] **Milestone 4 — `IrohTransport` landed** (`core/src/iroh.rs`, behind
+            the `iroh-transport` feature, off by default so the UniFFI/xcframework/
+            agent builds are untouched). Implements the `Transport` trait, emits
+            `MeshEvent`s; gossip carries clipboard `Message`s + presence over the
+            room-derived topic. Verified: `cargo run --features iroh-transport
+            --example iroh_engine` → the **real Engine** (unchanged public API)
+            syncs a clipboard A→B, E2E-decrypted, **no SSE/RTC/LAN/relay**. The
+            seam paid off — zero engine/client changes.
+            - [ ] Remaining: rendezvous for zero-config bootstrap (today via
+                  `TETHER_IROH_BOOTSTRAP` env stopgap); file transfer via
+                  `iroh-blobs`; make webrtc/mdns/reqwest optional so the iroh/wasm
+                  build is lean; A/B vs RTC+LAN; flip the feature on by default.
 - [ ] Spike **Automerge** as the Sync tier; re-express the clipboard as a tiny
       app over replicated state to validate the App API.
 - [ ] Durable device keypairs + authenticated handshake (Noise/`snow`, or
diff --git a/core/Cargo.toml b/core/Cargo.toml
index 792aedf..6fbba14 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -29,10 +29,24 @@ bytes = "1.12.0"
 chacha20poly1305 = "0.10.1"
 base64 = "0.22.1"
 getrandom = "0.4.3"
+# M4: the iroh Mesh, behind the `iroh-transport` feature (off by default, so the
+# UniFFI/xcframework/agent builds are untouched). When on, IrohTransport replaces
+# the hand-rolled SSE/RTC/LAN stack.
+iroh = { version = "1", optional = true }
+iroh-gossip = { version = "0.101", optional = true }
+
+[features]
+default = []
+iroh-transport = ["dep:iroh", "dep:iroh-gossip"]
 
 [dev-dependencies]
-# Iroh spike (examples/iroh_spike.rs): evaluate iroh as an alternative Mesh —
-# QUIC P2P + relay holepunch + key-based identity. Dev-only; not in the shipped lib.
+# Iroh spikes (examples/iroh_*.rs) link iroh directly regardless of the feature.
 iroh = "1"
 iroh-gossip = "0.101"
 anyhow = "1"
+
+# The M4 integration example drives the real Engine over IrohTransport, so it
+# needs the feature (and Engine::iroh_id, which only exists under it).
+[[example]]
+name = "iroh_engine"
+required-features = ["iroh-transport"]
diff --git a/core/examples/iroh_engine.rs b/core/examples/iroh_engine.rs
new file mode 100644
index 0000000..7041ecb
--- /dev/null
+++ b/core/examples/iroh_engine.rs
@@ -0,0 +1,88 @@
+//! M4 integration test — the real `Engine` syncing a clipboard over iroh.
+//!   cargo run --features iroh-transport --example iroh_engine
+//!
+//! Two engines, same account → same room → same gossip topic. No SSE/RTC/LAN:
+//! with the iroh-transport feature the engine's only transport is IrohTransport.
+//! Proves the public API (Engine::new/start/send + MessageHandler) is unchanged
+//! while the entire Mesh underneath is iroh — the payoff of the MeshEvent seam.
+//!
+//! Bootstrap is wired in-process via the TETHER_IROH_BOOTSTRAP env var (the v1
+//! stopgap until the rendezvous supernode): start A, read its EndpointId, hand
+//! it to B.
+
+use std::sync::mpsc;
+use std::time::{Duration, Instant};
+
+use tethercore::{Engine, Message, MessageHandler};
+
+struct Noop;
+impl MessageHandler for Noop {
+    fn on_message(&self, _m: Message) {}
+    fn on_status(&self, _c: bool) {}
+    fn on_file(&self, _n: String, _m: String, _d: Vec<u8>) {}
+}
+
+struct Collector(mpsc::Sender<String>);
+impl MessageHandler for Collector {
+    fn on_message(&self, m: Message) {
+        let _ = self.0.send(m.text);
+    }
+    fn on_status(&self, _c: bool) {}
+    fn on_file(&self, _n: String, _m: String, _d: Vec<u8>) {}
+}
+
+fn main() {
+    let account = "pecord@gmail.com"; // same account → same room → same topic + key
+    let room = "iroh-engine-test";
+    let server = "unused"; // iroh ignores cfg.server; it uses n0 infra
+
+    // Opener (no bootstrap yet).
+    let a = Engine::new(
+        server.into(), room.into(), "iroh-a".into(), "macos".into(), "A".into(), account.into(),
+    );
+    a.start(Box::new(Noop));
+
+    // Wait for A's iroh endpoint to come online, then bootstrap B off it.
+    let a_id = {
+        let deadline = Instant::now() + Duration::from_secs(20);
+        loop {
+            if let Some(id) = a.iroh_id() {
+                break id;
+            }
+            if Instant::now() > deadline {
+                eprintln!("❌ A never came online");
+                std::process::exit(1);
+            }
+            std::thread::sleep(Duration::from_millis(200));
+        }
+    };
+    eprintln!("A online, id={a_id}");
+    std::env::set_var("TETHER_IROH_BOOTSTRAP", &a_id);
+
+    // Joiner (bootstraps to A via the env var its IrohTransport reads at start).
+    let (tx, rx) = mpsc::channel();
+    let b = Engine::new(
+        server.into(), room.into(), "iroh-b".into(), "ios".into(), "B".into(), account.into(),
+    );
+    b.start(Box::new(Collector(tx)));
+
+    eprintln!("waiting for gossip topic to form…");
+    std::thread::sleep(Duration::from_secs(8));
+
+    let payload = "clipboard synced over iroh 🎉";
+    eprintln!("A sending: {payload:?}");
+    a.send(payload.to_string());
+
+    match rx.recv_timeout(Duration::from_secs(10)) {
+        Ok(t) if t == payload => {
+            println!("✅ engine-over-iroh: B received {t:?} (E2E-decrypted, via gossip, no relay/SSE/RTC/LAN)");
+        }
+        Ok(t) => println!("⚠️  B got {t:?} (expected {payload:?})"),
+        Err(_) => {
+            println!("❌ B received nothing — gossip topic may not have formed");
+            std::process::exit(1);
+        }
+    }
+    a.stop();
+    b.stop();
+}
diff --git a/core/src/iroh.rs b/core/src/iroh.rs
new file mode 100644
index 0000000..d18c8f8
--- /dev/null
+++ b/core/src/iroh.rs
@@ -0,0 +1,150 @@
+//! IrohTransport — the Mesh, on iroh. The end state of M4: this one transport
+//! subsumes the hand-rolled SSE/RTC/LAN stack. iroh handles key-addressing,
+//! holepunch, and relay; `iroh-gossip` carries the clipboard `Message`s and
+//! presence over the account-derived topic.
+//!
+//! It implements the same `Transport` trait and emits the same `MeshEvent`s as
+//! the legacy transports, so the engine event loop and every client are
+//! unchanged — the whole point of the MeshEvent seam.
+//!
+//! Bootstrap note (v1): joining the gossip topic needs ≥1 peer id. Until the
+//! rendezvous supernode lands (see ARCHITECTURE §M3), bootstrap ids come from
+//! the `TETHER_IROH_BOOTSTRAP` env var (comma-separated). That's the one
+//! stopgap; everything else is production-shaped.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, Mutex as StdMutex};
+
+use futures_util::TryStreamExt;
+use iroh::{endpoint::presets, protocol::Router, Endpoint, EndpointId};
+use iroh_gossip::{
+    api::{Event, GossipSender},
+    net::{Gossip, GOSSIP_ALPN},
+    proto::TopicId,
+};
+use tokio::runtime::Handle;
+
+use crate::{Config, EventTx, MeshEvent, Message, Transport};
+
+pub(crate) struct IrohTransport {
+    cfg: Config,
+    events: EventTx,
+    /// Set once the gossip topic is joined; used by `publish`.
+    sender: Arc<StdMutex<Option<GossipSender>>>,
+    /// This node's iroh EndpointId, surfaced once online (for bootstrap wiring).
+    id: Arc<StdMutex<Option<String>>>,
+}
+
+impl IrohTransport {
+    pub(crate) fn new(cfg: Config, events: EventTx, id: Arc<StdMutex<Option<String>>>) -> Self {
+        Self { cfg, events, sender: Arc::new(StdMutex::new(None)), id }
+    }
+}
+
+/// account/room → 32-byte gossip topic (consistent with room_for_account: the
+/// room is already sha256(account)[..4], this is the full-width derivation).
+fn topic_for(room: &str) -> TopicId {
+    use sha2::{Digest, Sha256};
+    let mut id = [0u8; 32];
+    id.copy_from_slice(&Sha256::digest(format!("tether:{room}").as_bytes()));
+    TopicId::from_bytes(id)
+}
+
+fn bootstrap_ids() -> Vec<EndpointId> {
+    std::env::var("TETHER_IROH_BOOTSTRAP")
+        .ok()
+        .into_iter()
+        .flat_map(|s| {
+            s.split(',')
+                .filter_map(|x| x.trim().parse::<EndpointId>().ok())
+                .collect::<Vec<_>>()
+        })
+        .collect()
+}
+
+impl Transport for IrohTransport {
+    fn name(&self) -> &'static str {
+        "iroh"
+    }
+
+    fn direct(&self) -> bool {
+        true // iroh chooses direct-vs-relay itself; treat as a direct peer channel
+    }
+
+    fn start(self: Arc<Self>, rt: Handle, running: Arc<AtomicBool>) {
+        let this = self;
+        rt.spawn(async move {
+            let endpoint = match Endpoint::bind(presets::N0).await {
+                Ok(e) => e,
+                Err(e) => {
+                    eprintln!("tethercore[iroh]: bind failed: {e}");
+                    return;
+                }
+            };
+            endpoint.online().await;
+            let my_id = endpoint.id().to_string();
+            *this.id.lock().unwrap() = Some(my_id.clone());
+            eprintln!("tethercore[iroh]: online id={my_id}");
+
+            let gossip = Gossip::builder().spawn(endpoint.clone());
+            let router = Router::builder(endpoint.clone())
+                .accept(GOSSIP_ALPN, gossip.clone())
+                .spawn();
+
+            let topic = topic_for(&this.cfg.room);
+            let sub = match gossip.subscribe_and_join(topic, bootstrap_ids()).await {
+                Ok(s) => s,
+                Err(e) => {
+                    eprintln!("tethercore[iroh]: topic join failed: {e}");
+                    return;
+                }
+            };
+            let (sender, mut receiver) = sub.split();
+            *this.sender.lock().unwrap() = Some(sender);
+            let _ = this.events.send(MeshEvent::Status(true));
+            eprintln!("tethercore[iroh]: joined topic {topic}");
+
+            while running.load(Ordering::SeqCst) {
+                match receiver.try_next().await {
+                    Ok(Some(Event::Received(msg))) => {
+                        if let Ok(m) = serde_json::from_slice::<Message>(&msg.content) {
+                            if m.from != this.cfg.from {
+                                let _ = this.events.send(MeshEvent::Message(m));
+                            }
+                        }
+                    }
+                    Ok(Some(Event::NeighborUp(id))) => {
+                        let _ = this.events.send(MeshEvent::DirectUp(id.to_string()));
+                    }
+                    Ok(Some(Event::NeighborDown(id))) => {
+                        let _ = this.events.send(MeshEvent::DirectDown(id.to_string()));
+                    }
+                    Ok(Some(_)) => {}        // Lagged, etc.
+                    Ok(None) => break,       // stream ended
+                    Err(e) => {
+                        eprintln!("tethercore[iroh]: recv error: {e}");
+                        break;
+                    }
+                }
+            }
+            drop(router);
+        });
+    }
+
+    fn publish(&self, rt: &Handle, msg: Message) {
+        // Clipboard + receipts ride gossip; other kinds are local-only.
+        if !(msg.kind.is_empty() || msg.kind == "clipboard" || msg.kind == "receipt") {
+            return;
+        }
+        let sender = self.sender.lock().unwrap().clone();
+        if let Some(sender) = sender {
+            let body = match serde_json::to_vec(&msg) {
+                Ok(b) => b,
+                Err(_) => return,
+            };
+            rt.spawn(async move {
+                let _ = sender.broadcast(body.into()).await;
+            });
+        }
+    }
+}
diff --git a/core/src/lib.rs b/core/src/lib.rs
index c547f49..200daa9 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -13,6 +13,12 @@
 //! across all transports and de-duplicates inbound so the same clipboard
 //! arriving over two channels surfaces once.
 
+// With the iroh-transport feature, the legacy SSE/RTC/LAN transports aren't
+// constructed (IrohTransport replaces them); they still compile, so quiet the
+// expected dead-code noise. Fully dropping their heavy deps (webrtc/mdns) for
+// the iroh/wasm build is a follow-up.
+#![cfg_attr(feature = "iroh-transport", allow(dead_code))]
+
 use std::collections::{HashMap, HashSet, VecDeque};
 use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use std::sync::{Arc, Mutex};
@@ -24,6 +30,8 @@ use tokio::runtime::Handle;
 use tokio::sync::mpsc::{UnboundedReceiver, UnboundedSender};
 
 mod crypto;
+#[cfg(feature = "iroh-transport")]
+mod iroh;
 mod lan;
 mod rtc;
 
@@ -333,6 +341,9 @@ pub struct Engine {
     /// Current message handler, swapped in on each `start()` (latest wins, as
     /// before) and read by the event loop per event.
     handler: Arc<Mutex<Option<Arc<dyn MessageHandler>>>>,
+    /// This node's iroh EndpointId once online (for bootstrap wiring in tests).
+    #[cfg(feature = "iroh-transport")]
+    iroh_id: Arc<Mutex<Option<String>>>,
 }
 
 #[uniffi::export]
@@ -367,6 +378,18 @@ impl Engine {
         // all state and reacts. Transports get a sender at construction; the
         // engine keeps one clone alive so the loop never sees the channel close.
         let (events_tx, events_rx) = tokio::sync::mpsc::unbounded_channel::<MeshEvent>();
+
+        // M4: with the iroh-transport feature, one iroh-backed transport replaces
+        // the hand-rolled SSE/RTC/LAN stack. Default build is unchanged.
+        #[cfg(feature = "iroh-transport")]
+        let iroh_id: Arc<Mutex<Option<String>>> = Arc::new(Mutex::new(None));
+        #[cfg(feature = "iroh-transport")]
+        let transports: Vec<Arc<dyn Transport>> = vec![Arc::new(iroh::IrohTransport::new(
+            cfg.clone(),
+            events_tx.clone(),
+            iroh_id.clone(),
+        ))];
+        #[cfg(not(feature = "iroh-transport"))]
         let transports: Vec<Arc<dyn Transport>> = vec![
             Arc::new(SseTransport {
                 cfg: cfg.clone(),
@@ -393,6 +416,8 @@ impl Engine {
             events_rx: Mutex::new(Some(events_rx)),
             loop_started: AtomicBool::new(false),
             handler: Arc::new(Mutex::new(None)),
+            #[cfg(feature = "iroh-transport")]
+            iroh_id,
         })
     }
 
@@ -634,6 +659,16 @@ impl Engine {
     }
 }
 
+/// Non-FFI helpers (iroh feature only) — used by the M4 integration example to
+/// wire bootstrap between two in-process engines.
+#[cfg(feature = "iroh-transport")]
+impl Engine {
+    /// This node's iroh EndpointId once the transport is online, else None.
+    pub fn iroh_id(&self) -> Option<String> {
+        self.iroh_id.lock().unwrap().clone()
+    }
+}
+
 /// Short-window de-dup so a clipboard echoed over both SSE and the RTC data
 /// channel surfaces once. Keyed on (from, text) — NOT ts: the SSE copy carries
 /// the server's stamp while the data-channel copy has none, so ts must be