If you don't care about dynamic hair or cloth, turning your friends into a Poser Figure or the bizzarre materials room go with P4 with PP. I personally perfer the old prop based hair because it was simpler to use. In fact, in my opinion, P5 is much more complicated and has very poor documentation. I'm still waiting for a good 3rd party book that can help me master the advanced features. If you plan to do most of your rendering in Poser, then the rendering engine might be the determining factor for you. The P5 rendering engine, Firefly, produces much more realistic skin tones and can do ray tracing, than the old P4 PP engine. To me the P5 human figures leave something to be desired, but some people like them.