PHP5 has better support for classic object-oriented style programming. I’m sure there are other advantages, but that’s the one I’ve heard most. Of course, we’re onto something like 5.2 or 5.3 at this point, so if you’re still on PHP4 you should probably make the leap. AFAIK, you can keep programming in PHP4 style in PHP5. Upgrading would allow you to start working with the newer features as you are comfortable with them.