Bump to v4.0.0.

internal/indexKeys.js

@@ -0,0 +1,22 @@
+define(['./baseTimes', '../isArguments', '../isArray', '../isLength', '../isString'], function(baseTimes, isArguments, isArray, isLength, isString) {
+
+  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
+  var undefined;
+
+  /**
+   * Creates an array of index keys for `object` values of arrays,
+   * `arguments` objects, and strings, otherwise `null` is returned.
+   *
+   * @private
+   * @param {Object} object The object to query.
+   * @returns {Array|null} Returns index keys, else `null`.
+   */
+  function indexKeys(object) {
+    var length = object ? object.length : undefined;
+    return (isLength(length) && (isArray(object) || isString(object) || isArguments(object)))
+      ? baseTimes(length, String)
+      : null;
+  }
+
+  return indexKeys;
+});