A group can be affected only by the ones on either side of it - so your boots, grouped to left and right feet, can be affected by the shins and toes, but not the thighs. The rule here would be, if bending the knees needs to deform the boots, you'll need shin groups too. I don't know V4, but it would depend on how close to the knees they get. If in doubt, include shin groups, I guess. The advantage to having everything in one group is that morphs are considerably simplified.