What advantages does this offer over P4? And can you still by this as a stand alone?

It is an add on to Poser 4. It gives you the possibility to use Python script (great advantage imo). You can also see thru more than one camera simultaniously, you can use animated textures, there's a Setup Room that will help you in character creation and Pro Pack uses png for thumbnails (it still reads the rsr's, then makes a copy of them as a png) and more important, it uses jpg bumpmaps, so you won't need any more HUGE .bum files. And finally it uses the obj files in Geometries without the need to create a geometry-rsr. As those geometry-rsr's are highly likely to get corrupted from time to time, I find that they're VERY nice to get rid of. Unless you specifically want the dynamics in P5, I'd recommend Pro Pack instead of P5 (I know that wasn't what you asked, but...) I have both Pro Pack and P5 but I rarely use P5, it is simply too slow on my system (lack of Ram, I know...)