A long time ago I had to solve the problem of running PHP4 and PHP5 on the same machine.
After a bit of searching I came across a posting in the german PHP-Mailing-List at http://www.phpbar.de where Norbert Pfeiffer described a possible way.
I choose to adapt my apachectl-file so that not one single instance of the apache-webserver was started but 2 instances. One of which listened on port 80, the other one on port 81.
This of course requires a unique httpd.conf for each of the instances.