Bump to v4.0.0.

internal/stackHas.js

@@ -0,0 +1,20 @@
+define(['./assocHas'], function(assocHas) {
+
+  /**
+   * Checks if a stack value for `key` exists.
+   *
+   * @private
+   * @name has
+   * @memberOf Stack
+   * @param {string} key The key of the entry to check.
+   * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
+   */
+  function stackHas(key) {
+    var data = this.__data__,
+        array = data.array;
+
+    return array ? assocHas(array, key) : data.map.has(key);
+  }
+
+  return stackHas;
+});