I've lurked for awhile and I'll throw my two scant cents in....

To me, V4 is very worth it. Why? She bends better & morphs better. People say you can't "un-V4" her face which is not entirely true, just requires some effort.
I find that characters for V3 pretty much look the same after awhile, I think I've seen the same shaped nose & lips too many times to count.
Gen 3 & Gen 2 females have, what I would call, "monkey-faces"...they tend to go flat when you morph any ethnic features, no depth, no noses that stick out like real people's do, & flat uber-smooched up lips. That too can be worked on IF one puts the effort in. I've done it with SP3's god-awful default horse face just using the morph dials.
Expressions are another matter, I hate trying V3's, V4's are much much better.

It would really depend on what you want to use your figures for & how comfortable you are with them.
I use V4 the most because I like her the most, but I break out the older figures as well, I like them too.
Each has it's use & unique charm.

I would say buy V4, try her out, if you don't like her before 30 days are up from purchase, get your money back & be happy with what you have. :)