There’s a FAQ about this one. Fedora Core 4 doesn’t provide PHP4, and so we’ve had to provide one for you. The release this weekend will add installation and setup automatically.
In short, use yum to install php4 and related packages. Then move /etc/httpd/conf.d/php.conf to /etc/httpd/conf.d/php.conf.disabled, and restart the httpd service. I think restarting Webmin is the quickest way to have it pickup the new php version.