You are going to have to pretty much change the extensions on all your pages from .php or whatever it is now (.php3 for me) to whatever extension is mapped to php4 (normally .php4). If you have administrator rights to the box you could simply remap all the .php3 pages to use the php4 engine, but if you are like me, you are leasing space and have no control over it.
If someone else has a better way please tell us, I just went through this (voluntarily, was extremely pleased that we got php4).
Jim Hawley