On HowToForge.com today, there's a new tutorial on getting suPHP to work with both PHP4 and PHP5 installations.
This tutorial shows how to install and use suPHP with PHP4 and PHP5. suPHP is a tool for executing PHP scripts with the permissions of their owners instead of the Apache user. With the help of suPHP you can also have PHP4 and PHP5 installed at the same time which I will describe at the end of this article.
The tutorial steps you through the process - installation of apache (with apt-get), disabling the built-in PHP, installing apxs, installation of PHP4-CGI, and, finally suPHP. From there, it's all just configuration of the different packages up to a finished, working install.