Congratulations on the BP M4!
Its my favorite M, has been from the beginning (my Leica mentor had one) and always will be.
As for the brass parts... early production cameras had an all brass/BP rewind part assemble. This was changed later to an alloy material that was black anodized (center flip-out post stayed brass painted black), so the later version outer part will show silver when the black wares through. The levers on front are BP brass. The lever caps are BP alloy on most cameras so will show silver, but some (very few) I've seen have brass caps. The shutter speed dial is black anodized alloy, unlike the earlier BP M2 or M3 which where brass/BP.
Enjoy your camera 😀