I suppose V4 is popular because she is so well supported.  Every one knew when V4 was released that she was going to be the most popular poser figure. Her popularity means that, she will be well supported, thus she will become even more popular.  It's self re-inforcing to a degree.

Having said that, I do think V4 is a great mesh over all. She does bend quite well compared to some of the other figures out there and she looks attractive (for a 3d figure).

For me the biggest drawback is that V4 is quite a complex figure. I mean all those JMC's and what not's make it pretty difficult for a novice (like me) to make clothing for her. Hacking cr2 files, to make sure that the clothing confirms relatively well with V4, is a pretty daunting task.  There's something very odd and complex about how her cr2 file works.

D'oh! Why do things that happen to stupid people keep happening to me?