Bump to v4.0.0.

internal/cloneMap.js

@@ -0,0 +1,17 @@
+import addMapEntry from './addMapEntry';
+import arrayReduce from './arrayReduce';
+import mapToArray from './mapToArray';
+
+/**
+ * Creates a clone of `map`.
+ *
+ * @private
+ * @param {Object} map The map to clone.
+ * @returns {Object} Returns the cloned map.
+ */
+function cloneMap(map) {
+  var Ctor = map.constructor;
+  return arrayReduce(mapToArray(map), addMapEntry, new Ctor);
+}
+
+export default cloneMap;