V4 - been around for ages and there's lots of content available for her free and paid.  Downloading the free V4WM and Outfitter scripts means you have a very well done weight mapped figure that bends and moves as well as any other weight mapped figure, with a free conversion program to weight map clothing you have already.

V6 - works in DS only unless you install DSON in Poser and then is slow and cumbersome to use in Poser.

Dawn - limited clothing and characters available at this point just because she was just released on August 9.  Given time, she may have as much available as V4.

So, if you're just starting out, there's not much difference between the figures.

It just depends on 1) personal preference for software - Poser or DS and 2) personal preference for which figure you find most useful.

If you have a heavy investment in V4 already, then I suspect you'll find yourself going back to V4 over and over again.