I'm still using P4 and ProPack at the moment as it does everything I need (along with all the extra goodies I've bought and downloaded from Renderosity, of course!). I did try P5 briefly but didn't bother to upgrade as I'm happy with my current setup. There are some nice new features in P5 but as many have already mentioned, they are things that maybe you won't use all the time. Also, I still prefer the P4 render engine over the newer P5 "firefly" one. But I guess that's down to personal taste. I'm sure you'll have lots of fun whatever version you use but don't think P4 is obsolete just because it's not the latest edition. There are still lots of users out there!