Agree that the M4 will likely have a small edge in weight and suspension dynamics to improve its lap time (ala M5 vs. M6), but the engines will be identical (maybe slight tune variation with the turbos).
We already know the M3/4 will be lighter and more powerful than the current E9x, and from the looks of the 4 series concept, an amazing design evolution. The outlook is extremely positive for the M3/M4 variants.
My only concern is turbo lag in the new 6 cyl. FI engine, M will have to deliver magic in this respect. I've driven a X6M and it has lag without a doubt.