1.) V3 has a version of Daz's "Muscular" morphs!  V4 has no such morphs.  For a look at these morphs click on the "Homepage" link under my avatar, and/or the lips in my signature  The male figure is Daz3D's M2, and the female is DAZ's V3.

2.)  V3 is higher resolution, which although translates to higher resource requirements, means sharper images.

3.) I'll wager there is more V3 stuff for free than there is at sale for V4!  All in all V3 has like10 times more stuff than V4, and it will probably be that way for another year or two, maybe more...

4.) And...(Drumroll please!)...Victoria 3 has been Free, for a long, long time!  Go to download.com, and search for "DAZ" you'll see a Figure pak with V3 in it(...As well as Bryce 5and it's Presets!)  You'll still need to purchase her head and body morph packs, but these are probably dirt cheap through the platinum club, and do go on sale now and again.