What makes V4 so popular?

At this point in time, nothing more than inertia. Vendors have had 8 years to work out a quicky (for them) workflow, have learned the gotcha's of P4 level tech, and gotten into a rut, for lack of a more politic way to put it. Users have nigh onto a decade of content they've purchased and downloaded, and frankly are probably scared to start over (as they did with each previous figure from the supplier) and be forced to figure out just how much they've spent over time on that one figure. Neither state is sustainable for much longer. But people are scared.

Moving on means learning new things. Tossing aside old hacks that aren't needed anymore. Changing methodologies. Classic conservative mentality run amuck; It's new, It's different, I don't wanna....

It's not that the old figures are 'bad'. Just the methodologies used in them are seriously outdated. Mesh doesn't have an expiration date; it can be rerigged, upgraded, and be quite useable with more modern systems (one reason I'm trying to teach myself rigging and weightmapping; lots of mesh in the library.... :P ).

New figures get slammed because a success there would destroy the status quo. But time is on the side of the innovators. Within a year or so, we'll have 3 generations of Poser with the newer tech. Since the split with DAZ, the reasons to enshrine the P4 era tech are gone, gone, gone. The g-thing does not work natively in Poser, so it is essentially irrelevant. New users tend to get the latest version....which means P9-10. So there is a slow, but inevitable growth based solely around what the modern application is capable of. The wise vendors have been getting their toes wet for awhile now; if standard business ideas apply, those who do not adapt and move forward will be crushed when the old paradigm collapses. It is going to happen, and those who haven't moved forward will get left behind and have a doubly hard task trying to catch up.

Oh, Coleman, I'd disagree. Poser is the motorcycle; Max and Lightwave and Cinema are the packed out sports cars. And considering that this little low cost tool of ours is being used to make the likes of RWBY, I'd say it's a fairly powerful little tool for its size (Ghods I want that Optitrack system so bad.....) ;D

As always, it's not the size of the tool, it's the skill of the tool user.