I think this is why they are still considering both M3 Coupe and M4.
As someone already meantioned there is a lot of history tied to the M3 name, you might lose some people with the name change, "I drive a M3" is not the same as "I drive a M4". Personally I think they will keep the M3 name for the coupe. The Gran Coupe will then perfectly adopt the M4 name and be the only version called the M4 and have the price premium to go with it.
However, one reason they could call the coupe an M4 is that they could argue they are keeping tradition and the M3 name alive by deciding to make the M3 Sedan (even though at first rumors were this would not be made), its almost like they made the sedan version just to keep the M3 name alive!
I will be in the market for a M3 Sedan or M4 Gran Coupe so I don't really care what they call the 4 series M Coupe version
Also Note that Mercedes is still calling their AMG cars "63" even though they have the 5.5L engines now. Naming cars is all about marketing now, not what makes the most sense. And for that reason the M3 Coupe will probably live on.