No ErrorDocument 404
won't work on your way out from PHP. That is only applicable when Apache detects 404 for an incoming request and ends up invoking ErrorDocument 404
handler.
Once control is handed over to PHP as normal request processor Apache just returns output returned by PHP module to a requesting client.
Only thing you can do is this:
require_once("404.php"); // include 404 handler
exit;
And inside 404.php
you can do:
http_response_code(404); // sends 404 status to browser