My experience (G4 700, 1GB RAM) is that P4 is much quicker to pose in, particularly if the figure has lots of dials. In P5 I'd often get the spinning beachball. I mostly used P5 anyways, because of better renders, no bum-files that fill up the hard disk, better materials etc etc. Sometimes I would start posing the figure in P4 and then bring it into P5. Recently I got a G5 and Poser 5 is behaving quite nicely on it :)