For what it's worth, EP4 version 4.0.36.ZC (available for download from https://www.zen-cart.com/downloads.php?do=file&id=2069) has been tested against ZC 1.5.5d with no issues; however, there has been a reported issue with ZC 1.5.5d and a change introduced in versions c and d that may need to be undone in admin/includes/functions/database.php as referenced in this post although that issue did not appear in my testing against product addition or modification either manually or with EP4.