Hi,
So you have an issue with a third party component or a server configuration.
If I call directly the "pxpay_4.php" file (without any single parameter) ; I got a 404 page.
If I call the notification URL, I got a blank page (the result we expected)
index.php?option=com_hikashop&ctrl=checkout&task=notify¬ify_payment=pxpay¬if_id=4&tmpl=component&lang=en
You can see to activate the system plugin "hikashop payment" ; it should catch the calls for the Hikashop payment notifications.
But if the problem is above that (like server configuration), we won't be able to help you.
The goal is simple, you should be able to call that PHP file and get a blank page (instead of a 404).
Regards,