Well, can you create files on P5 that are compatible with P4 (and pp)(if you don't use the face/hair/clothes rooms) or is everything saved in P5 not compatible?

Yes, they'll work in Poser 4 and/or Pro Pack. Another element to consider is the Material Room; procedural shaders and other new material features don't work in P4. For maximum compatibility, you should edit the files manually, changing the version numbers to avoid the "newer version than expected" message.

Ah, thanks Dragon. Now, lets say i use procedural shaders in a .pz3 file from P5 and i edit it to change the version number, will it load in P4 with just no material or something or will there be other problems? (all this if mainly thinking on exporting the .pz3 files to Vue 4 and C4D btw)

It should load successfully into P4, but if you're using a procedural texture (for instance), you'll be left with nothing but the object colour. The same goes for any shader tree structure. I was fooling around with the Toon shader node last year, saved the figure to the Library, and then decided to see if it would still load into Pro Pack. The figure loaded fine, but it was blank white, since the textures were plugged into the shader, instead of plugged directly into the Diffuse_Color channel.