Bump to v4.0.0.

once.js

@@ -0,0 +1,25 @@
+define(['./before'], function(before) {
+
+  /**
+   * Creates a function that is restricted to invoking `func` once. Repeat calls
+   * to the function return the value of the first invocation. The `func` is
+   * invoked with the `this` binding and arguments of the created function.
+   *
+   * @static
+   * @memberOf _
+   * @category Function
+   * @param {Function} func The function to restrict.
+   * @returns {Function} Returns the new restricted function.
+   * @example
+   *
+   * var initialize = _.once(createApplication);
+   * initialize();
+   * initialize();
+   * // `initialize` invokes `createApplication` once
+   */
+  function once(func) {
+    return before(2, func);
+  }
+
+  return once;
+});