From what I understand: 

V4 "Unimesh" = uses the same mesh topology for female, male, child etc figures. Has nothing to do with rigging methods.

Poser Unimesh = cuts up the figure into body parts upon creation, then stitches them back up so that things like Subdivision can work. Doesn't make any difference for end users, but for content creators this adds more steps that we have to be careful for when creating things, also complicates bringing HD (subdivided) morphs into Poser, and brings a bunch of bugs when attempting to add some features into the rigging.

(edit: I'm not on the payroll, what I'm saying is not official, just the info I've gathered from experience. Could be wrong, etc.)

True Unimesh = doesn't cut up the figure at all. I'd wager the dev team is having trouble bringing this into Poser because it would change a very core functionality of it that's been around ever since Poser exists.

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Feel free to call me Ohki!

Poser Pro 11, Poser 12 and Poser 13, Windows 10, Superfly junkie. My units are milimeters.

Persephone (the computer): AMD Ryzen 9 5900x, RTX 3070 GPU, 96gb ram.