There's a very interesting note in John Coggeshall's wiki that many will probably be looking for as PHP5 looms on the horizion - Running PHP 4 and PHP 5 Concurrently (and yes, on the same box...heh)

On Coggeshall.org, I wanted to setup PHP 4 and PHP 5. I had hoped that all of my old code would work in PHP 5 still, but there are a few applications out there such as the Gallery photo album which are really very incompatible with PHP 5.

To solve this problem, I used Apache's mod_proxy module and two apache servers... My primary server runs PHP 5 as an Apache module and a second apache server runs PHP 4.3.5 as a module. My configuration requires that you can setup multiple virtual servers and run Apache to listen on localhost on port 8080. Here's what I did...

The wiki entry gives a pretty brief entry to get it all set up, but it's the result of hours of work on the topic - it's always easy once you've done it...