My guess on why there will be a gran coupe version of the 4er is because the 6 has already been announced having a gran coupe vers. The 4er, in my opinion, will follow the same marketing strategy as the 6er. 4 -> smaller 6 in all respects. 3er is in fact very similiar in styling to 5. Expect the same lines, and tail lights in the 4er that are currently on the 6. SPECULATION.
"I like the look of the 6er gran coupe but the price is out of my reach; oh, look, they make a 4er gran coupe"
2006 M3: Individual Blue Onyx on Black. MT