V4 is better because it's newer. Newer always equals Better. Well, if you buy into the DAZ philosophy that is.
V4 does have some advantages. IMO, it's UV mapped better, and the morph forms for body and hands and eyes save alot of time, especially for the hands which can be tedious, posing all 16 parts individually.

M4, OTOH, IMO,  just sucks, period. Terrible design, goofy looking bastard, like they put as minimal effort into it as possible. All kinds of problems.