diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 1a7e20e0..ac8399c9 100755
--- a/lib/content/conformance.sh
+++ b/lib/content/conformance.sh
@@ -15,7 +15,7 @@ if [ ! -x "$SX_SERVER" ]; then
   fi
 fi
 
-SUITES=(block doc render api store crdt)
+SUITES=(block doc render api store crdt sync)
 
 OUT_JSON="lib/content/scoreboard.json"
 OUT_MD="lib/content/scoreboard.md"
@@ -44,6 +44,7 @@ run_suite() {
 (load "lib/content/api.sx")
 (load "lib/content/store.sx")
 (load "lib/content/crdt.sx")
+(load "lib/content/sync.sx")
 (epoch 2)
 (eval "(define content-test-pass 0)")
 (eval "(define content-test-fail 0)")
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index 490e10f4..3c580915 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -5,9 +5,10 @@
     "render": {"pass": 29, "fail": 0},
     "api": {"pass": 26, "fail": 0},
     "store": {"pass": 29, "fail": 0},
-    "crdt": {"pass": 34, "fail": 0}
+    "crdt": {"pass": 34, "fail": 0},
+    "sync": {"pass": 14, "fail": 0}
   },
-  "total_pass": 196,
+  "total_pass": 210,
   "total_fail": 0,
-  "total": 196
+  "total": 210
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index fc7fac98..7fb590aa 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -10,4 +10,5 @@ _Generated by `lib/content/conformance.sh`_
 | api | 26 | 0 | 26 |
 | store | 29 | 0 | 29 |
 | crdt | 34 | 0 | 34 |
-| **Total** | **196** | **0** | **196** |
+| sync | 14 | 0 | 14 |
+| **Total** | **210** | **0** | **210** |
diff --git a/lib/content/sync.sx b/lib/content/sync.sx
new file mode 100644
index 00000000..45b6ac60
--- /dev/null
+++ b/lib/content/sync.sx
@@ -0,0 +1,71 @@
+;; content-on-sx — external CMS sync via an injected adapter.
+;;
+;; Sync is a peripheral, not a feature. The core defines a SHAPE — an adapter is
+;; a dict {:import (fn external doc-id -> doc) :export (fn doc -> external)} — and
+;; delegates to it. The core knows nothing about Ghost's data model; all
+;; translation lives in the adapter. Swap the adapter and the core is unchanged;
+;; if Ghost goes away, nothing here does.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+;; ── generic boundary: pure delegation ──
+(define
+  content/import
+  (fn (adapter external doc-id) ((get adapter :import) external doc-id)))
+
+(define content/export (fn (adapter doc) ((get adapter :export) doc)))
+
+;; round-trip a document through an adapter (export then import).
+(define
+  content/round-trip
+  (fn
+    (adapter doc)
+    (content/import adapter (content/export adapter doc) (doc-id doc))))
+
+;; ── a Ghost-flavoured adapter (the peripheral). Ghost knowledge is confined
+;; here: a post is {:title :sections (list section)}; a section is a tagged dict
+;; {:kind ...} that this adapter maps to/from content blocks. ──
+(define
+  ghost-section->block
+  (fn
+    (sec)
+    (let
+      ((kind (get sec :kind)) (id (get sec :id)))
+      (cond
+        ((= kind "heading")
+          (mk-heading id (get sec :level) (get sec :text)))
+        ((= kind "paragraph") (mk-text id (get sec :text)))
+        ((= kind "image") (mk-image id (get sec :src) (get sec :alt)))
+        ((= kind "code") (mk-code id (get sec :language) (get sec :text)))
+        ((= kind "quote") (mk-quote id (get sec :cite) (get sec :text)))
+        ((= kind "hr") (mk-divider id))
+        ((= kind "list") (mk-list id (get sec :ordered) (get sec :items)))
+        ((= kind "embed") (mk-embed id (get sec :url) (get sec :provider)))
+        (else (mk-text id (get sec :text)))))))
+
+(define
+  block->ghost-section
+  (fn
+    (b)
+    (let
+      ((t (blk-type b)) (id (blk-id b)))
+      (cond
+        ((= t "heading") {:id id :text (str (blk-send b "text")) :kind "heading" :level (blk-send b "level")})
+        ((= t "text") {:id id :text (str (blk-send b "text")) :kind "paragraph"})
+        ((= t "image") {:id id :src (str (blk-send b "src")) :alt (str (blk-send b "alt")) :kind "image"})
+        ((= t "code") {:id id :text (str (blk-send b "text")) :kind "code" :language (str (blk-send b "language"))})
+        ((= t "quote") {:cite (str (blk-send b "cite")) :id id :text (str (blk-send b "text")) :kind "quote"})
+        ((= t "divider") {:id id :kind "hr"})
+        ((= t "list") {:items (blk-send b "items") :id id :kind "list" :ordered (blk-send b "ordered")})
+        ((= t "embed") {:id id :provider (str (blk-send b "provider")) :kind "embed" :url (str (blk-send b "url"))})
+        (else {:id id :text "" :kind "paragraph"})))))
+
+(define
+  ghost-import
+  (fn
+    (post doc-id)
+    (doc-new doc-id (map ghost-section->block (get post :sections)))))
+
+(define ghost-export (fn (doc) {:sections (map block->ghost-section (doc-blocks doc))}))
+
+(define ghost-adapter {:export ghost-export :import ghost-import})
diff --git a/lib/content/tests/sync.sx b/lib/content/tests/sync.sx
new file mode 100644
index 00000000..fa87cb19
--- /dev/null
+++ b/lib/content/tests/sync.sx
@@ -0,0 +1,74 @@
+;; Phase 4 — external CMS sync via injected adapter. Import/export round-trip.
+
+(st-bootstrap-classes!)
+(content-bootstrap-blocks!)
+(content-bootstrap-doc!)
+(content-bootstrap-render!)
+
+;; ── a Ghost post (external shape) ──
+(define post {:sections (list {:id "h" :text "Hello" :kind "heading" :level 1} {:id "p" :text "World" :kind "paragraph"} {:id "i" :src "/c.png" :alt "cat" :kind "image"} {:id "d" :kind "hr"} {:items (list "a" "b") :id "l" :kind "list" :ordered true}) :title "Hello"})
+
+;; ── import (delegates to adapter) ──
+(define doc (content/import ghost-adapter post "post"))
+(content-test "import doc-id" (doc-id doc) "post")
+(content-test "import ids" (doc-ids doc) (list "h" "p" "i" "d" "l"))
+(content-test
+  "import types"
+  (doc-types doc)
+  (list "heading" "text" "image" "divider" "list"))
+(content-test
+  "import renders"
+  (content/render doc "html")
+  "<h1>Hello</h1><p>World</p><img src=\"/c.png\" alt=\"cat\"><hr><ol><li>a</li><li>b</li></ol>")
+(content-test
+  "import preserves heading level"
+  (blk-send (doc-find doc "h") "level")
+  1)
+(content-test
+  "import preserves list items"
+  (blk-send (doc-find doc "l") "items")
+  (list "a" "b"))
+
+;; ── export (delegates to adapter) ──
+(define out (content/export ghost-adapter doc))
+(content-test
+  "export sections round-trip"
+  (get out :sections)
+  (get post :sections))
+
+;; ── round-trip: export then import yields the same document ──
+(define doc2 (content/round-trip ghost-adapter doc))
+(content-test "round-trip ids" (doc-ids doc2) (doc-ids doc))
+(content-test
+  "round-trip render"
+  (content/render doc2 "html")
+  (content/render doc "html"))
+
+;; ── round-trip the external form: import . export . import == import ──
+(content-test
+  "external round-trip sections"
+  (get
+    (content/export ghost-adapter (content/import ghost-adapter post "post"))
+    :sections)
+  (get post :sections))
+
+;; ── core knows nothing about Ghost: a different (stub) adapter works the same ──
+(define raw-adapter {:export (fn (d) (str (blk-send (doc-find d "only") "text"))) :import (fn (ext doc-id) (doc-new doc-id (list (mk-text "only" ext))))})
+(define rdoc (content/import raw-adapter "just text" "r"))
+(content-test "alt adapter import" (doc-ids rdoc) (list "only"))
+(content-test
+  "alt adapter export"
+  (content/export raw-adapter rdoc)
+  "just text")
+
+;; ── code / quote / embed kinds round-trip ──
+(define post2 {:sections (list {:id "c" :text "(+ 1 2)" :kind "code" :language "sx"} {:cite "Ada" :id "q" :text "to err" :kind "quote"} {:id "e" :provider "vimeo" :kind "embed" :url "https://v/1"})})
+(define d3 (content/import ghost-adapter post2 "p2"))
+(content-test
+  "code/quote/embed types"
+  (doc-types d3)
+  (list "code" "quote" "embed"))
+(content-test
+  "code/quote/embed round-trip"
+  (get (content/export ghost-adapter d3) :sections)
+  (get post2 :sections))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index 2d8904cd..d203bbc4 100644
--- a/plans/content-on-sx.md
+++ b/plans/content-on-sx.md
@@ -19,7 +19,7 @@ injected adapter, not core.
 
 ## Status (rolling)
 
-`bash lib/content/conformance.sh` → **196/196** (Phases 1–3 complete: blocks, doc, render, api, persist op log, CRDT merge)
+`bash lib/content/conformance.sh` → **210/210** (Phases 1–3 complete + Phase 4 Ghost adapter)
 
 ## Ground rules
 
@@ -71,12 +71,20 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] concurrent-edit tests (any order, double-apply → identical)
 
 ## Phase 4 — External sync + federation
-- [ ] Ghost/CMS sync via injected adapter (import/export)
+- [x] Ghost/CMS sync via injected adapter (import/export)
 - [ ] federated documents (peer-authored blocks) — trust-gated stub
-- [ ] tests: round-trip import/export, conflict on concurrent external edit
+- [~] tests: round-trip import/export (done), conflict on concurrent external edit (pending)
 
 ## Progress log
 
+- 2026-06-07 — Phase 4 `sync.sx` (cb1): external CMS sync via an injected
+  adapter. Core defines the shape — `{:import :export}` — and delegates;
+  `content/import` / `content/export` / `content/round-trip` know nothing about
+  Ghost. A Ghost-flavoured adapter confines all format translation (post
+  `:sections` ↔ content blocks, all 8 kinds). Swapping in a stub `raw-adapter`
+  works identically. Round-trip (export∘import and import∘export) preserves ids,
+  types, fields, order. 14 tests; suite 210/210. Next: trust-gated federation +
+  concurrent-external-edit conflict (via CRDT).
 - 2026-06-07 — Phase 3 `crdt.sx` (**Phase 3 complete**): collaborative merge as
   a state-based CvRDT. Merge is a join (lub) on a semilattice → commutative,
   associative, idempotent by construction. Ordering = unique dense Logoot