I think that Poser four will complain about poser 5 files ... not sure if they work 100% or not. Haven't tried. I have both installed, with Poser5 able to find the poser 4 libraries. I build characters and scenes in four as much as I can, since I'm more used to it. And then switch over to 5 for the finishing touches and render. It seems to work so far :)