diff --git a/lib/ocaml/eval.sx b/lib/ocaml/eval.sx
index 778e0d16..58c323dd 100644
--- a/lib/ocaml/eval.sx
+++ b/lib/ocaml/eval.sx
@@ -782,6 +782,55 @@
                             (begin
                               (set! env (ocaml-env-extend env name v))
                               (set! result (merge result (dict name v))))))))))
+                ((= tag "def-mut")
+                  ;; let x = ... and y = ... — sequential top-level binds.
+                  (let ((bs (nth decl 1)))
+                    (begin
+                      (define run-one
+                        (fn (b)
+                          (let ((nm (nth b 0)) (ps (nth b 1)) (rh (nth b 2)))
+                            (let ((v (if (= (len ps) 0)
+                                         (ocaml-eval rh env)
+                                         (ocaml-make-curried ps rh env))))
+                              (begin
+                                (set! env (ocaml-env-extend env nm v))
+                                (set! result (merge result (dict nm v))))))))
+                      (define loop
+                        (fn (xs)
+                          (when (not (= xs (list)))
+                            (begin (run-one (first xs)) (loop (rest xs))))))
+                      (loop bs))))
+                ((= tag "def-rec-mut")
+                  ;; let rec f = ... and g = ... — mutual recursion.
+                  (let ((bs (nth decl 1)) (cells (list)))
+                    (begin
+                      (define alloc
+                        (fn (xs)
+                          (when (not (= xs (list)))
+                            (let ((b (first xs)))
+                              (let ((c (list nil)) (nm (nth b 0)))
+                                (begin
+                                  (append! cells c)
+                                  (set! env (ocaml-env-extend env nm
+                                              (fn (a) ((nth c 0) a))))
+                                  (alloc (rest xs))))))))
+                      (alloc bs)
+                      (let ((idx 0))
+                        (begin
+                          (define fill
+                            (fn (xs)
+                              (when (not (= xs (list)))
+                                (let ((b (first xs)))
+                                  (let ((nm (nth b 0)) (ps (nth b 1)) (rh (nth b 2)))
+                                    (let ((v (if (= (len ps) 0)
+                                                 (ocaml-eval rh env)
+                                                 (ocaml-make-curried ps rh env))))
+                                      (begin
+                                        (set-nth! (nth cells idx) 0 v)
+                                        (set! result (merge result (dict nm v)))
+                                        (set! idx (+ idx 1))
+                                        (fill (rest xs)))))))))
+                          (fill bs))))))
                 ((= tag "expr")
                   (ocaml-eval (nth decl 1) env))
                 ((= tag "module-def")
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 3b5edbef..8499f101 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1172,6 +1172,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4703)
 (eval "(ocaml-run \"((1 + 2) : int) * 3\")")
 
+;; ── Module body: def-mut / def-rec-mut ─────────────────────────
+(epoch 4800)
+(eval "(ocaml-run-program \"module M = struct let rec a n = if n = 0 then 0 else b (n - 1) and b n = if n = 0 then 1 else a (n - 1) end ;; M.a 5\")")
+(epoch 4801)
+(eval "(ocaml-run-program \"module M = struct let x = 1 and y = 2 end ;; M.x + M.y\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1857,6 +1863,10 @@ check 4701 "let f (x : int) : int"        '42'
 check 4702 "(5 : int)"                    '5'
 check 4703 "((1+2) : int) * 3"            '9'
 
+# ── Module body: def-mut / def-rec-mut ─────────────────────────
+check 4800 "module rec a/b mutual"        '1'
+check 4801 "module x and y"               '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index db6ce1b4..0ebacfc1 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,10 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 4 — `:def-mut` / `:def-rec-mut` inside module
+  bodies (+2 tests, 475 total). `ocaml-eval-module` now handles
+  multi-binding `let .. and ..` decls. `module M = struct let rec a n =
+  ... and b n = ... end` works.
 - 2026-05-08 Phase 5.1 — bfs.ml baseline (20/20 pass). Graph
   breadth-first search using Queue + Hashtbl visited-set + List.assoc_opt
   + List.iter. Returns the count of reachable nodes (6 for the demo