Bump to v4.0.0.

flatten.js

@@ -0,0 +1,21 @@
+import baseFlatten from './internal/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) : [];
+}
+
+export default flatten;