Bump to v4.0.0.

internal/mapGet.js

@@ -1,14 +1,23 @@
+import Map from './Map';
+import assocGet from './assocGet';
+import hashGet from './hashGet';
+import isKeyable from './isKeyable';
+
 /**
- * Gets the cached value for `key`.
+ * Gets the map value for `key`.
  *
  * @private
  * @name get
- * @memberOf _.memoize.Cache
+ * @memberOf MapCache
  * @param {string} key The key of the value to get.
- * @returns {*} Returns the cached value.
+ * @returns {*} Returns the entry value.
  */
 function mapGet(key) {
-  return key == '__proto__' ? undefined : this.__data__[key];
+  var data = this.__data__;
+  if (isKeyable(key)) {
+    return hashGet(typeof key == 'string' ? data.string : data.hash, key);
+  }
+  return Map ? data.map.get(key) : assocGet(data.map, key);
 }
 
 export default mapGet;