I've had php4 running with apache2 for a while now and wanted to get php5 also simultaneously running. There are other ways to do this, but I opted with getting php5 to run as fastcgi and keep php4 as apache2 module so they can co-exist together.
This was done on CentOS release 4.5 with ISPConfig Control Panel on a dedicated hosting scenario. If you're looking to share the server with others then you might want to set up with suphp/SuExec for security purposes.
