To use MadelineProto 4.0 w/ async, you have to load the latest version of MadelineProto from the master branch by loading it through composer (dev-master) or with madeline.php:

<?php

if (!file_exists('madeline.php')) {
copy('<a href="https://phar.madelineproto.xyz/madeline.php">https://phar.madelineproto.xyz/madeline.php</a>', 'madeline.php');
}
define('MADELINE_BRANCH', '');
include 'madeline.php';


In a few weeks I will set MadelineProto 4.0 as default with madeline.php: in the meantime, I do not provide support for the old version.