If you need PHP4 on a CentOS5/RHEL5 server then you’ll find the RPMs linked below very handy. They install to /opt/php4 and can coexist with a default PHP 5 install.
The included conf.d/php4.conf file includes sample lines to run it as a CGI or as a module.
RPMs are here:
http://repo.conforge.com/conforge/CentOS/5/i386/RPMS/
http://repo.conforge.com/conforge/CentOS/5/x86_64/RPMS/
See this URL for details to add the yum repo:
http://wiki.conforge.com/wiki/Linux-PHP4cgiAndPHP5on+CentOS5