Bump to v4.0.0.

internal/toFunction.js

@@ -0,0 +1,14 @@
+import identity from '../identity';
+
+/**
+ * Converts `value` to a function if it's not one.
+ *
+ * @private
+ * @param {*} value The value to process.
+ * @returns {Function} Returns the function.
+ */
+function toFunction(value) {
+  return typeof value == 'function' ? value : identity;
+}
+
+export default toFunction;