Just 4. 

Poser 4 and 5 mostly just for testing these days.  I suspect Poser 5's days are numbered, though.

 Poser 6 is mostly what I work in, actually have 3 copies on two computers (I have to totally clean out a copy of Poser when I test a new product, to make sure no files are missing from the Zip, one copy is just for that). 

I don't use Poser 7 much yet, but it is on two computers, too.   I haven't had any trouble with it at all, though. 

ProPack is gone, though.