If you want to run both php4 and php5 in apache, you must install one of them to run as a module (the default) and the other to run as a cgi. Read
this howto to see how this can be done. It's written for Debian and Ubuntu but the configuration steps are quite the same.
You can then put in the <VirtualHost>...</VirtualHost> part of the site in question the following (assuming that you installed php4 as a cgi):
Code:
ScriptAlias /php-cgi /usr/lib/php4/bin/php-cgi
AddHandler php-script .php4 .php-cgi
Action php-script /cgi-bin/php4