I highly doubt that they will change the name of the M3 coupe. I have seen plenty of other articles that say the 4 series designation will be for the coupes HOWEVER... the M3 coupe will still retain the name M3. I really dont think BMW could afford to change the name of the M3 coupe as it is their flagship. I think they know that doing so would anger allot of enthusiast as well as put a blemish on the whole essence of the car. I know that sounds like BS but it is true. They can play with all the other car names (1M, X6M, ect..) but they can not mess with the M3 name.
I can just see it now:
M3 Driver: Hey I drive an M4
Random Joe: What is that?
M3 Driver:It is the new M3 coupe
Random Joe:Oh, well its still not an M3.
M3 Driver: (Smack, crash, bang) Say that again, I dare you!