said by adsldude:Upgrading from PHP4 to PHP5 should not be a big deal. The current scripts are working on PHP4 so they should work just fine on PHP5. Now if anybody starts to use some of the new PHP5 only features after you upgrade then you'll be SOL for falling back to PHP4.
I recently upgraded all my dedicated servers to PHP5 and didn't miss a beat. Good luck!
Cpanel/WHM allows you to run Php4 and php 5.
For some of my websites, I left them at php4 because I'm too lazy to fix them for php5