Originally Posted by TK-421
Because you're messing with history, tradition. The 5 and 6 look absolutely nothing alike while the 3 series coupe/sedan/vert are the same car.
But the 3 series sedan and 4 series coupe are said to look more different from each other than ever in the next generation.
So might be some more justification for BMW for the 3/4 m3/m4 split.