diff --git a/lib/mod/conformance.conf b/lib/mod/conformance.conf
index 5e5e0921..dc704baa 100644
--- a/lib/mod/conformance.conf
+++ b/lib/mod/conformance.conf
@@ -16,10 +16,12 @@ PRELOADS=(
   lib/mod/lifecycle.sx
   lib/mod/audit.sx
   lib/mod/api.sx
+  lib/mod/fed.sx
 )
 
 SUITES=(
   "decide:lib/mod/tests/decide.sx:(mod-decide-tests-run!)"
   "audit:lib/mod/tests/audit.sx:(mod-audit-tests-run!)"
   "escalation:lib/mod/tests/escalation.sx:(mod-escalation-tests-run!)"
+  "fed:lib/mod/tests/fed.sx:(mod-fed-tests-run!)"
 )
diff --git a/lib/mod/fed.sx b/lib/mod/fed.sx
new file mode 100644
index 00000000..855f71f8
--- /dev/null
+++ b/lib/mod/fed.sx
@@ -0,0 +1,145 @@
+;; lib/mod/fed.sx — federation: cross-instance reports, decision sharing, trust,
+;; revocation. fed-sx itself is mocked here (an in-memory outbox); the real wire
+;; transport would replace mod/fed-send!.
+;;
+;; Trust is advisory by default (the hard rule): a peer's decision only binds
+;; locally when (mod/trusted? peer :mod) holds. An untrusted peer's decision is
+;; recorded as a suggestion in the advisory log and is NOT applied. Local
+;; decisions propagate outward via the outbox. Revocation undoes a locally
+;; applied action when its proof is invalidated, notifying the origin peer.
+
+(define mod/*fed-trust* (list))     ;; {:peer :scope}
+(define mod/*fed-outbox* (list))    ;; {:to :type :payload}
+(define mod/*fed-advisory* (list))  ;; {:peer :decision} — received, not applied
+(define mod/*fed-applied* (list))   ;; {:report-id :action :origin :revoked}
+(define mod/*fed-origins* (list))   ;; {:id :origin}
+
+(define
+  mod/fed-reset!
+  (fn
+    ()
+    (begin
+      (set! mod/*fed-trust* (list))
+      (set! mod/*fed-outbox* (list))
+      (set! mod/*fed-advisory* (list))
+      (set! mod/*fed-applied* (list))
+      (set! mod/*fed-origins* (list)))))
+
+;; ── trust model ──
+
+(define
+  mod/trust-match?
+  (fn
+    (t peer scope)
+    (if (= (get t :peer) peer) (= (get t :scope) scope) false)))
+
+(define
+  mod/grant-trust
+  (fn (peer scope) (begin (append! mod/*fed-trust* {:scope scope :peer peer}) true)))
+
+(define
+  mod/revoke-trust
+  (fn
+    (peer scope)
+    (set!
+      mod/*fed-trust*
+      (reduce
+        (fn
+          (acc t)
+          (if (mod/trust-match? t peer scope) acc (append acc (list t))))
+        (list)
+        mod/*fed-trust*))))
+
+(define
+  mod/trusted?
+  (fn
+    (peer scope)
+    (mod/any? (fn (t) (mod/trust-match? t peer scope)) mod/*fed-trust*)))
+
+;; ── cross-instance reports ──
+
+(define
+  mod/fed-receive-report
+  (fn
+    (peer by about reason)
+    (let
+      ((r (mod/report by about reason)))
+      (begin (append! mod/*fed-origins* {:id (mod/report-id r) :origin peer}) r))))
+
+(define
+  mod/report-origin
+  (fn
+    (id)
+    (reduce
+      (fn (acc o) (if (= (get o :id) id) (get o :origin) acc))
+      "local"
+      mod/*fed-origins*)))
+
+;; ── decision sharing (mock fed-sx send) ──
+
+(define
+  mod/fed-send!
+  (fn (to type payload) (begin (append! mod/*fed-outbox* {:type type :to to :payload payload}) true)))
+
+(define mod/fed-outbox (fn () mod/*fed-outbox*))
+
+(define
+  mod/fed-share-decision
+  (fn
+    (decision peers)
+    (reduce
+      (fn
+        (acc p)
+        (begin (mod/fed-send! p "decision" decision) (append acc (list p))))
+      (list)
+      peers)))
+
+;; ── receiving a peer's decision (advisory unless trusted) ──
+
+(define
+  mod/fed-applied-action
+  (fn
+    (report-id)
+    (reduce
+      (fn (acc a) (if (= (get a :report-id) report-id) a acc))
+      nil
+      mod/*fed-applied*)))
+
+(define
+  mod/fed-receive-decision
+  (fn
+    (peer decision)
+    (if
+      (mod/trusted? peer :mod)
+      (begin (append! mod/*fed-applied* {:revoked false :action (get decision :action) :report-id (get decision :report-id) :origin peer}) {:advisory false :peer peer :applied true :decision decision})
+      (begin (append! mod/*fed-advisory* {:peer peer :decision decision}) {:advisory true :peer peer :applied false :decision decision}))))
+
+;; ── revocation ──
+
+(define
+  mod/fed-revoke!
+  (fn
+    (report-id reason)
+    (begin
+      (set!
+        mod/*fed-applied*
+        (map
+          (fn (a) (if (= (get a :report-id) report-id) {:revoked true :action (get a :action) :report-id (get a :report-id) :origin (get a :origin)} a))
+          mod/*fed-applied*))
+      (mod/fed-send! (mod/report-origin report-id) "revocation" {:report-id report-id :reason reason})
+      report-id)))
+
+;; re-run the engine; if the action no longer holds, the prior decision's proof
+;; is invalidated — revoke the applied moderation.
+(define
+  mod/fed-revoke-if-invalidated
+  (fn
+    (report decision reports rules)
+    (let
+      ((d2 (mod/decide-report report reports rules)))
+      (if
+        (= (get d2 :action) (get decision :action))
+        {:revoked false :decision d2}
+        (begin
+          (mod/fed-revoke! (get decision :report-id) "proof invalidated")
+          {:revoked true :decision d2})))))
diff --git a/lib/mod/scoreboard.json b/lib/mod/scoreboard.json
index 50d894b5..3ebb8e38 100644
--- a/lib/mod/scoreboard.json
+++ b/lib/mod/scoreboard.json
@@ -1,12 +1,13 @@
 {
   "lang": "mod",
-  "total_passed": 106,
+  "total_passed": 132,
   "total_failed": 0,
-  "total": 106,
+  "total": 132,
   "suites": [
     {"name":"decide","passed":31,"failed":0,"total":31},
     {"name":"audit","passed":29,"failed":0,"total":29},
-    {"name":"escalation","passed":46,"failed":0,"total":46}
+    {"name":"escalation","passed":46,"failed":0,"total":46},
+    {"name":"fed","passed":26,"failed":0,"total":26}
   ],
-  "generated": "2026-06-06T17:49:32+00:00"
+  "generated": "2026-06-06T17:54:02+00:00"
 }
diff --git a/lib/mod/scoreboard.md b/lib/mod/scoreboard.md
index c063d904..108e0273 100644
--- a/lib/mod/scoreboard.md
+++ b/lib/mod/scoreboard.md
@@ -1,9 +1,10 @@
 # mod scoreboard
 
-**106 / 106 passing** (0 failure(s)).
+**132 / 132 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | decide | 31 | 31 | ok |
 | audit | 29 | 29 | ok |
 | escalation | 46 | 46 | ok |
+| fed | 26 | 26 | ok |
diff --git a/lib/mod/tests/fed.sx b/lib/mod/tests/fed.sx
new file mode 100644
index 00000000..0dc73bd2
--- /dev/null
+++ b/lib/mod/tests/fed.sx
@@ -0,0 +1,154 @@
+;; lib/mod/tests/fed.sx — Phase 4: federation (mock fed-sx).
+
+(define mod-fed-count 0)
+(define mod-fed-pass 0)
+(define mod-fed-fail 0)
+(define mod-fed-failures (list))
+
+(define
+  mod-fed-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! mod-fed-count (+ mod-fed-count 1))
+      (if
+        (= got expected)
+        (set! mod-fed-pass (+ mod-fed-pass 1))
+        (begin
+          (set! mod-fed-fail (+ mod-fed-fail 1))
+          (append!
+            mod-fed-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+(mod/reset!)
+(mod/fed-reset!)
+
+;; ── trust model (advisory by default) ──
+
+(mod-fed-test! "trust initially false" (mod/trusted? "peerA" :mod) false)
+(mod/grant-trust "peerA" :mod)
+(mod-fed-test! "trust after grant" (mod/trusted? "peerA" :mod) true)
+(mod-fed-test! "trust wrong scope" (mod/trusted? "peerA" :other) false)
+(mod-fed-test! "trust other peer" (mod/trusted? "peerB" :mod) false)
+(mod/revoke-trust "peerA" :mod)
+(mod-fed-test! "trust after revoke" (mod/trusted? "peerA" :mod) false)
+
+;; ── cross-instance reports ──
+
+(define
+  mod-fed-fr
+  (mod/fed-receive-report "peerB" "alice" "bob" "this is spam"))
+(mod-fed-test! "fed report assigned id r1" (mod/report-id mod-fed-fr) "r1")
+(mod-fed-test! "fed report origin is peer" (mod/report-origin "r1") "peerB")
+(define mod-fed-local (mod/report "carol" "dave" "fine post"))
+(mod-fed-test!
+  "local report origin is local"
+  (mod/report-origin (mod/report-id mod-fed-local))
+  "local")
+(mod-fed-test!
+  "engine decides fed report (spam → hide)"
+  (get
+    (mod/decide-report mod-fed-fr (list mod-fed-fr) mod/default-rules)
+    :action)
+  "hide")
+
+;; ── decision sharing (outbox) ──
+
+(define mod-fed-dec {:action "hide" :rule "spam-hide" :report-id "r1"})
+(define
+  mod-fed-shared
+  (mod/fed-share-decision mod-fed-dec (list "peerB" "peerC")))
+(mod-fed-test! "share returns notified peers" (len mod-fed-shared) 2)
+(mod-fed-test! "outbox has two messages" (len (mod/fed-outbox)) 2)
+(mod-fed-test!
+  "outbox message type decision"
+  (get (first (mod/fed-outbox)) :type)
+  "decision")
+(mod-fed-test!
+  "outbox message addressed to peer"
+  (get (first (mod/fed-outbox)) :to)
+  "peerB")
+
+;; ── receiving a peer decision: advisory unless trusted ──
+
+(define mod-fed-untrusted (mod/fed-receive-decision "peerZ" {:action "remove" :rule "reviewer-remove" :report-id "rx"}))
+(mod-fed-test!
+  "untrusted decision not applied"
+  (get mod-fed-untrusted :applied)
+  false)
+(mod-fed-test!
+  "untrusted decision advisory"
+  (get mod-fed-untrusted :advisory)
+  true)
+(mod-fed-test!
+  "untrusted decision absent from applied log"
+  (mod/fed-applied-action "rx")
+  nil)
+(mod-fed-test!
+  "advisory log records suggestion"
+  (len mod/*fed-advisory*)
+  1)
+
+(mod/grant-trust "peerT" :mod)
+(define mod-fed-trusted (mod/fed-receive-decision "peerT" {:action "hide" :rule "spam-hide" :report-id "ry"}))
+(mod-fed-test! "trusted decision applied" (get mod-fed-trusted :applied) true)
+(mod-fed-test!
+  "trusted decision binds locally"
+  (get (mod/fed-applied-action "ry") :action)
+  "hide")
+
+;; ── revocation ──
+
+(mod-fed-test!
+  "applied action not yet revoked"
+  (get (mod/fed-applied-action "ry") :revoked)
+  false)
+(mod/fed-revoke! "ry" "manual")
+(mod-fed-test!
+  "revoke marks applied action revoked"
+  (get (mod/fed-applied-action "ry") :revoked)
+  true)
+(mod-fed-test!
+  "revoke emits a revocation message"
+  (mod/any? (fn (m) (= (get m :type) "revocation")) (mod/fed-outbox))
+  true)
+
+;; revoke-if-invalidated: proof still holds → no revocation
+(define mod-fed-spam-r (mod/mk-report "rs" "a" "b" "this is spam"))
+(define
+  mod-fed-spam-d
+  (mod/decide-report mod-fed-spam-r (list mod-fed-spam-r) mod/default-rules))
+(mod-fed-test! "spam decision is hide" (get mod-fed-spam-d :action) "hide")
+(define
+  mod-fed-rev-same
+  (mod/fed-revoke-if-invalidated
+    mod-fed-spam-r
+    mod-fed-spam-d
+    (list mod-fed-spam-r)
+    mod/default-rules))
+(mod-fed-test!
+  "valid proof → not revoked"
+  (get mod-fed-rev-same :revoked)
+  false)
+
+;; exoneration invalidates the proof → revocation
+(define
+  mod-fed-exon-r
+  (mod/attach-evidence mod-fed-spam-r (mod/mk-evidence "exonerated" "mod")))
+(define
+  mod-fed-rev-inv
+  (mod/fed-revoke-if-invalidated
+    mod-fed-exon-r
+    mod-fed-spam-d
+    (list mod-fed-exon-r)
+    mod/default-rules))
+(mod-fed-test!
+  "invalidated proof → revoked"
+  (get mod-fed-rev-inv :revoked)
+  true)
+(mod-fed-test!
+  "re-decision after exoneration is keep"
+  (get (get mod-fed-rev-inv :decision) :action)
+  "keep")
+
+(define mod-fed-tests-run! (fn () {:failures mod-fed-failures :total mod-fed-count :passed mod-fed-pass :failed mod-fed-fail}))
diff --git a/plans/mod-on-sx.md b/plans/mod-on-sx.md
index 01821c18..8634b8d6 100644
--- a/plans/mod-on-sx.md
+++ b/plans/mod-on-sx.md
@@ -16,7 +16,7 @@ federation extension.
 
 ## Status (rolling)
 
-`bash lib/mod/conformance.sh` → **106/106** (Phases 1–3 complete)
+`bash lib/mod/conformance.sh` → **132/132** (Phases 1–4 complete — roadmap done)
 
 ## Ground rules
 
@@ -113,14 +113,37 @@ lib/mod/fed.sx
 
 ## Phase 4 — Federation
 
-- [ ] cross-instance reports — peer raises report about local content (or vice versa)
-- [ ] decision sharing — actions taken locally propagate to peers via fed-sx
-- [ ] trust model — peer's decision is advisory unless `(trust peer :mod)` is granted
-- [ ] revocation — undo applied moderation if proof was invalidated
-- [ ] `lib/mod/tests/fed.sx` — federated decision chains (mock fed-sx in tests)
+- [x] cross-instance reports — `mod/fed-receive-report peer …` ingests a peer's
+  report into the local registry, tagging origin; `mod/report-origin` resolves it
+  (local reports default to `"local"`); the engine decides federated reports
+  unchanged
+- [x] decision sharing — `mod/fed-share-decision decision peers` pushes messages to
+  the mock outbox (`mod/fed-send!` is the seam the real fed-sx transport replaces)
+- [x] trust model — `mod/fed-receive-decision` applies a peer's decision locally
+  ONLY when `(mod/trusted? peer :mod)`; otherwise it lands in the advisory log,
+  unapplied. `mod/grant-trust` / `mod/revoke-trust` manage the trust registry
+- [x] revocation — `mod/fed-revoke!` marks the applied action revoked + emits a
+  revocation message to the origin; `mod/fed-revoke-if-invalidated` re-runs the
+  engine and revokes only when the action no longer holds (proof invalidated)
+- [x] `lib/mod/tests/fed.sx` — 26 cases: trust grant/scope/revoke, cross-instance
+  ingest + origin, outbox sharing, advisory-vs-trusted apply, revocation +
+  invalidation (exoneration flips hide→keep → revoked)
 
 ## Progress log
 
+- **Phase 4 complete — 132/132** (+26 fed). **Full roadmap done.** Federation:
+  cross-instance reports, decision sharing, advisory-by-default trust, revocation.
+  fed-sx is mocked behind `mod/fed-send!` (in-memory outbox) — the only seam a real
+  transport must replace. The hard rule is enforced: a peer's decision binds
+  locally only under `(mod/trusted? peer :mod)`; otherwise it is recorded as a
+  suggestion and never auto-applied. Revocation composes with the proof model from
+  Phase 2 — `mod/fed-revoke-if-invalidated` re-runs the *same* engine and undoes a
+  moderation only when the action it once proved no longer holds (an exoneration
+  evidence flips hide→keep, triggering revocation + an origin-bound revocation
+  message).
+  - **Liftable (acl-sx watch):** the trust registry (`grant`/`revoke`/`trusted?`
+    over `{:peer :scope}`) and the outbox/send! seam are generic federation
+    plumbing; candidates for `lib/guest/` if acl-sx grows a federation phase.
 - **Phase 3 complete — 106/106** (+46 escalation). Lifecycle state machine,
   auto/human tiers, appeal-override, and an api façade. The state machine is a
   pure SX module (`lib/mod/lifecycle.sx`) over the engine — policy stays in