Bump to v4.0.0.

internal/stackHas.js

@@ -0,0 +1,19 @@
+var assocHas = require('./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);
+}
+
+module.exports = stackHas;