Bump to v4.0.0.

uniqueId.js

@@ -0,0 +1,28 @@
+define(['./toString'], function(toString) {
+
+  /** Used to generate unique IDs. */
+  var idCounter = 0;
+
+  /**
+   * Generates a unique ID. If `prefix` is provided the ID is appended to it.
+   *
+   * @static
+   * @memberOf _
+   * @category Util
+   * @param {string} [prefix] The value to prefix the ID with.
+   * @returns {string} Returns the unique ID.
+   * @example
+   *
+   * _.uniqueId('contact_');
+   * // => 'contact_104'
+   *
+   * _.uniqueId();
+   * // => '105'
+   */
+  function uniqueId(prefix) {
+    var id = ++idCounter;
+    return toString(prefix) + id;
+  }
+
+  return uniqueId;
+});