Update 4.md

修正Object单词拼写错误

4.md

@@ -176,7 +176,7 @@ console.log(Object.keys({x: 0, y: 0, z: 2}));
 
 ```js
 let objectA = {a: 1, b: 2};
-bject.assign(objectA, {b: 3, c: 4});
+Object.assign(objectA, {b: 3, c: 4});
 console.log(objectA);
 // → {a: 1, b: 3, c: 4}
 ```