Bump to v4.0.0.

sum.js

@@ -0,0 +1,23 @@
+var baseSum = require('./internal/baseSum'),
+    identity = require('./identity');
+
+/**
+ * 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;
+}
+
+module.exports = sum;