Change your Server configuration, to have php4 handle php extensions.
For Apache, add (or update) those lines
8<------------------
AddType application/x-httpd-php4 .php4 .php
Action application/x-httpd-php4 "path_to_php4.exe"
------------------->8
Be sure to replace path_to_php4.exe with the real path to your php4 binary.
If your Webserver is not Apache, I can't help, sorry 🙁