You know what, I don't buy it either. BMW plays with their nomenclature so much these days (*ahem 550i is actually a 4.8L, wtf*) but really they don't do much with their liter-age. Obviously BMW know how to make a powerful engine thats efficient at the same time and dont have to go the benz route of just making the engines bigger and bigger in displacement. So my money is that BMW goes with the tried and true 4.0L size and would just play with different materials in engine construction in order to let the thing rev to a higher threshold, which is more of the type of car the M3 has become (more of a highstrung car rather then a big block cruiser).