It's my understanding that some people have used V4 in the P4 ProPack.  I, myself, have opened V4 in Poser 5 but don't have P4 installed any longer.  The one thing I get in P5 is the message about being a newer version than expected but she does open.

The reason they say P6 or better is because of the extensive use of the shader nodes that P4 doesn't have so you lose a lot of resolution on the skin textures.

Hugz from Phoenix, USA

Victoria

Remember, sometimes the dragon wins. Correction: MOST times.