Another thing not mentioned before.
I guess Vmcg may be lower for a quad, compared to a twin of a comparable size, due to a lower thrust generated by each engine (think A340 vs A330, 34000lbs vs 70000lbs per eng.) - this, combined with better acceleration capability may allow for a lower V1. Then again outboard engines on a quad are mounted on a longer arm, I don't know which has a stronger effect on Vmcg...