For now the only advantage Genesis offers is improved joint bending (which is also available for the Gen4 figures though various add on joint and musculature fixes) and the ability to mix her with the other basic Genesis shapes.
Well. for a couple more things -
Genesis’s improved joint bending also applies to Genesis’s clothes; using the any of the Perfect V4 or Ironman etc added JCM packs on V4 is all well and good (especially if you only do reneders of nudes), but the bending of the clothes V4 is wearing is not improved by the added V4 JCMs (and in most cases trying to transfer V4 specific JCMs to clothing with Morphing Clothes or the like is not going to help, and indeed is likely to make things worth). It’s not only that Genesis bends better than V4 or M4; Genesis’s clothes bend better than theirs too.
And then there’s the way parts of Genesis can be scaled. I’m just in the throws of creating a character with Genesis with the physique based on that of a certain real (female) person. Am still trying to choose whether to base on the V4, V5, V5SM, S5 or S5M shapes, but for all of them the shoulders aren’t broad enough for the physique I’m trying to recreate. With Genesis there’s a simple proportion dial to increase the shoulder width that just xscales chest and collars, and the figure works fine. Try applying the same xscaling to V4’s body parts and all the joints in her arms break ... very nasty. So the ability with Genesis to trivially move or scale some of the actual joint centres without then having to re-rig all the child bones makes Genesis more versatile in terms of the range of proportions of characters it can be used for.