diff --git a/.changeset/hip-seals-kiss.md b/.changeset/hip-seals-kiss.md
new file mode 100644
index 000000000..9d0ea32e8
--- /dev/null
+++ b/.changeset/hip-seals-kiss.md
@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Fix regression from #4639 where isComputedProp returned false for lazy @computed properties before first read
diff --git a/packages/mobx/__tests__/decorators_20223/stage3-decorators.ts b/packages/mobx/__tests__/decorators_20223/stage3-decorators.ts
index 00bc79b2b..39dc348e8 100644
--- a/packages/mobx/__tests__/decorators_20223/stage3-decorators.ts
+++ b/packages/mobx/__tests__/decorators_20223/stage3-decorators.ts
@@ -22,7 +22,8 @@ import {
     IAtom,
     createAtom,
     runInAction,
-    makeObservable
+    makeObservable,
+    isComputedProp
 } from "../../src/mobx"
 import { $mobx, type ObservableArrayAdministration } from "../../src/internal"
 import * as mobx from "../../src/mobx"
@@ -1191,6 +1192,19 @@ test("4616 - @computed decorator should be lazy", () => {
     expect(computeCount).toBe(0)
 })
 
+test("4616 - isComputedProp reports lazy @computed before first read", () => {
+    class Order {
+        @computed
+        get total() {
+            return 3
+        }
+    }
+
+    const o = new Order()
+
+    t.equal(isComputedProp(o, "total"), true)
+})
+
 test("4616 - observe on @computed before first read materialises it", () => {
     class Order {
         @observable accessor price: number = 3
diff --git a/packages/mobx/src/api/iscomputed.ts b/packages/mobx/src/api/iscomputed.ts
index 692da37e3..2a7d07e4c 100644
--- a/packages/mobx/src/api/iscomputed.ts
+++ b/packages/mobx/src/api/iscomputed.ts
@@ -7,7 +7,11 @@ export function _isComputed(value, property?: PropertyKey): boolean {
     if (isObservableObject(value) === false) {
         return false
     }
-    if (!value[$mobx].values_.has(property)) {
+    const adm = value[$mobx]
+    if (adm.lazyComputedKeys_?.has(property)) {
+        return true
+    }
+    if (!adm.values_.has(property)) {
         return false
     }
     const atom = getAtom(value, property)