I thought the deal was that the B6 is firmer than the B4, but otherwise similar?
I thought the B4 could hand 20-30mm of lowering, and therefore the B6 can too, but -
(1) If you want firmer and lower, then the B8s are the best option because they are firm like B6s but designed for lowering.
(2) You can get different bump stops for B4s and B6s to eliminate (or reduce?) the banging over potholes that can come from lowering.
Please correct me if I am wrong.