P4 morphs to V2P4? No, V2P4 has a higher vertex count. You could, however, do something like this with WarriorDL's Vicky head on Posette body figure (Polly?).

P3 morphs to P4? P4 characters are cut slightly differently around the problem joints than P3, making vertex counts incompatible for some body parts. Vertex order was changed for other parts, resulting in the "exploding head" syndrome. There may be a few body parts on certain characters where this works though.