Poser 4 was the breakthrough software.

Poser propack was P4 with some decient bug fixes. It also changed the bump format from bum files to jpg, which are much smaller and more universal to handel. Poser Artist is now P4pro.

P5 was P4 with some new rooms wratched onto it and tiny bit better render engine.

P6 was the final bug fixs for P5. It also had a tiny bit better lghting engine.

Basicly the differances your asking about have to do with bump maping and transparancies on the textures.