IMO there is no reason to get upset because of the name change.
The M3 will remain as sedan while the 4-series (that's what they should have called it from the beginning) will be the M4 Coupe. It makes more sense and it does not sound weird at all.
Forget about tradition and so on. People will talk about the M4 in 30 years just like we do now about the M3. They won't forget the M3 coupe, in contrast, that beautiful car will always be the father of all M-cars in everyone's mind. This step will even make the M3 more legendary while possibly creating another legend - the M4.