I think the M4/3 will show if BMW is leaving "The Ultimate Driving Machine" even more or not, the S55 points in a good direction in my eyes since that engine will combine the best of a natural aspirated and a turbo engine, but of course BMW isn't just about engines or M cars.
The next sign from BMW will be the M2/GC, I think it will be the true new M3 and there's only one generation left before you can forget it forever when it becomes pretty sure FWD. So BMW really should make that last one right and put a damn S55 with ca. 360 PS in it.
A 4cyl. just would be a punch in the face of all the true BMW fans out there who are willing to buy one of these, a N55 successor would be acceptable but not what you would expect from a M car.