I've been able to open Poser 5 pz3's in P4/PP. There is an error message that says its a newer version and poser will try to open it, then pretty much opens it. I haven't tried the dynamics from P5 in P4, but if its the type of scene file that only uses P4 items then everything seems fine. As for the rsr, P5 will read those. Linking the runtime library to the P4 library with rsr's in it works just fine.