Bump to v4.0.0.

flatten.js

@@ -0,0 +1,22 @@
+define(['./internal/baseFlatten'], function(baseFlatten) {
+
+  /**
+   * Flattens `array` a single level.
+   *
+   * @static
+   * @memberOf _
+   * @category Array
+   * @param {Array} array The array to flatten.
+   * @returns {Array} Returns the new flattened array.
+   * @example
+   *
+   * _.flatten([1, [2, 3, [4]]]);
+   * // => [1, 2, 3, [4]]
+   */
+  function flatten(array) {
+    var length = array ? array.length : 0;
+    return length ? baseFlatten(array) : [];
+  }
+
+  return flatten;
+});