HowtoForge have a tutorial on Debian Sarge and Ubuntu for running Apache2 with PHP5 and PHP4 at the same time
Enjoy it fully!
This tutorial shows how to install and configure Apache2 with PHP5 and PHP4 enabled at the same time. Because it is not possible to run both PHP5 and PHP4 as Apache modules, we must run one of them as CGI, the other one as Apache module. In this document I will use PHP5 as Apache module and PHP4 as CGI, and I will describe the setup for the Linux distributions Debian Sarge (3.1) and Ubuntu 5.10 (Breezy Badger).