V4 is a Poser thing. ?

DAZ originally made content for Poser. Their figures were widely used by Poser users - much more than the figures that come with Poser.

Eventually, DAZ created DAZ Studio, so they wouldn't be dependent on Poser. (TBH, there was some doubt about Poser's survival at times.) They were fairly compatible at first, but now the programs have diverged so much they're not that compatible any more.

A lot of Poser users still use V4, because it's not easy to get DAZ's newer figures into Poser. Poser has its own newer figures, like La Femme, but after all these years, there's a TON of content for V4, it's relatively cheap compared to newer stuff, and so a lot of people still use V4.

There's also mods like Sasha-16, which adds weight maps to V4 and improves her posing.