Bump to v4.0.0.

internal/stackClear.js

@@ -0,0 +1,15 @@
+define([], function() {
+
+  /**
+   * Removes all key-value entries from the stack.
+   *
+   * @private
+   * @name clear
+   * @memberOf Stack
+   */
+  function stackClear() {
+    this.__data__ = { 'array': [], 'map': null };
+  }
+
+  return stackClear;
+});