Bump to v4.0.0.

sum.js

@@ -0,0 +1,26 @@
+define(['./internal/baseSum', './identity'], function(baseSum, identity) {
+
+  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
+  var undefined;
+
+  /**
+   * Computes the sum of the values in `array`.
+   *
+   * @static
+   * @memberOf _
+   * @category Math
+   * @param {Array} array The array to iterate over.
+   * @returns {number} Returns the sum.
+   * @example
+   *
+   * _.sum([4, 2, 8, 6]);
+   * // => 20
+   */
+  function sum(array) {
+    return (array && array.length)
+      ? baseSum(array, identity)
+      : undefined;
+  }
+
+  return sum;
+});