If you will be working with dynamic clothing, neither figure offers any advantage over the other.  If you're modeling your own conforming clothing, then V4 requires quite a bit less work because you will need a lot fewer JCMs (joint controlled morphs).  Both figures have some pretty major anatomy problems (actually, pretty much all popular Poser have pretty major anatomy problems, but eh).  Nearly all the big commercial skin texturers have moved on from V3 to V4 so if you wanted new skin textures - which you're not asking about, I realize - then you could benefit there.

Short answer is if you're happy with V3 and will be working mostly with dynamics, you gain little or nothing from trying to convert to V4, particularly if you do your own clothing modeling.

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