The M4 if you look at it from BMW M's perspective is the opportunity to create a new iconic M car just as BMW will reflect the whole image of the 4er with a more exclusive sportier looking and driving car than before.
The M4 will be as different to the F80 as the F30 to the F32. Both cars will be tuned differently although sharing the same components , drivetrain and engine. The M4 will reflect the more dynamic side of the M family.
Do not think that BMW M do not know what has to be done here they know exactly where the current M3 is in status and that is stills at the top of its game where it has been since 2007.
And keeping that status even with added progression is the upmost priority for M's engineers.
The advent of the Avant only Audi RS 4 has got the water coolers in the FIZ buzzing and although nothing has appeared officially , there is much discussion regarding rumours about a possible M3 Touring finally becoming a reality since each generation has got close and closer yet to the green light only to evaporate at the end. Both Audi and Mercedes-Benz offer successful high performance wagons in that segment but BMW do not.
Much discussion about the renaming etc , on the Design night some journalists could not believe that BMW went through with it.
I suppose it would be fun walking down a street and seeing a new M4 or 4er with a 3er badge. So I can turn around to my wife and say "what a backwards person?"
I should mention that the rehetoric surrounding the 4er and M4 has been heard by everyone in BMW not only in the present but throughout each generation of cars. When the truth of the matter is for some its specifically status- the newest of the new. You will be in the next one. Does not matter if it is an M4 or a 440i.