4 cylinders make lots of sense.
I8's 1.5l, 3cyl. engine output is 77hp per cylinder. And that is a non M engine. A 4cyl. engine with the same specific output would make 308hp. Put in some of the M's "high response turbo" magic from the upcoming M4/M3 and it could easily climb toward 350 very responsive hp, or even more. I'd guess this engine could be all the A/CLA/GLA45AMG's engine isn't.
