I'd wait to see what Poser 10 brings to the table before I'd spend a lot of money.

Perhaps it'll make Genesis work much better in Poser or perhaps the new figures will be so good you don't even want to use V4 or V5 anymore.

But nethertheless, here are the Pros and Cons for V4 and V5 as I currently see it:

Pros:

V4: Lots and lots and lots of content. Several "tool kits" available to improve her rigging. Free weightmapped version available. Works very well natively in Poser. Injection system to reduce cr2 size. Can be mixed with G4, A4 and S4. Can wear V3 clothing using V4toV3 figure.

V5: Vastly improved rigging due to weightmapping. The new "It" girl, so she gets the latest, greatest stuff. Part of the Genesis Genepool, so she can be mixed with any other Genesis figure. More realistic default shape than V4. Can wear all existing 3rd and 4th gen clothes using legacy shapes. Can be expanded by geografting (Studio only) and other UV sets, so V4 textures still work. Has subdivision functionality.

 

Cons:

V4: Very poorly rigged. Lots of magnet "clutter" make her a memory hog, and the joint improvement kits even add to that problem (Free weightmapped version doesn't have that problems). Stylized default shape. Wastes polygons for LOD functionality that doesn't work in Poser, so less mesh detail than V3. Available joint improvements need clothing to be tweaked, too. Has been used for six years now, so people get a bit tired of her.

V5: Needs DSON importer to work in Poser which doesn't support all of her functionality. Very low mesh resolution, so not much built in mesh detail. (Smoothing is achieved via subdivision) No injection system in Poser, so cr2 files can get pretty big over time with huge loading times. Much less content available and new content coming at a much lesser rate as she needs Studio to make content for her.

 

So, I'd download Studio 4.5 to get the Genesis base and maybe buy V5 and download the DSON importer to have a direct comparison inside Poser.

But as I said I'd wait with content shopping until I know where Poser 10 is headed.