The "skins" (or rather "textures" in Poser lingo) belong to the individual model and not directly to the Poser version - so any texture for the "P3 posette" will work with exactly this model in P3 and P4. However, P4 has the feature to have different textures for individual body parts (while P3 only has one global texture for the whole body) - so most P4 models require P4 and won't work suitable with P3.