I have these, ordered through CSG. Recently installed. The BM4 is a mid-point between the basic GT and the GT-R calipers. The basic GT caliper is a two-piece design made by pinning/bolting two separately CNCed from castings pieces, the GT-R is CNCed monoblock caliper made from billet. The BM4 is CNCed from a casting, creating a monoblock caliper, but it is not from billet. I'm not sure the code, but Brembo also has two-pieces which are CNCed from billet and then pinned.
In any case, typically a monoblock caliper has less flex in the caliper body resulting in reduced pedal travel to reach a particular braking pressure and more consistent braking feel and they are more durable under hard use. Forged monoblock is the top end, but it's very pricey, the BM4 seemed to me a nice entry-point since my previous BBK was two-piece forged. Why forged vs cast matters, is essentially the same as with wheels (as both are unsprung mass), is that forged billet is lower weight for equal strength vs CNC or flow-formed castings. So the BM4 weighs more than the GT-R caliper of the same size, and the GT-R caliper is also theoretically stronger, but in both cases it is a monoblock caliper and should perform better than a two-piece design.
Yes, you can still install the pad from the back of the caliper by removing two pins and the spring clip, just as you can on any other Brembo caliper. A standard Brembo caliper pin tool/punch works fine on the BM4.
Picture so you can see that the normal pins are in place... installing pads is easy.
with a 17x9 wheel
