Originally Posted by Uli_HH
The reason for the special chassis code for the M3/M4 is to show that the M3 is not the top model of the 3er series ... the M3 is an sportscar only in an 3er outfit and no 3er.
I strongly doubt that the reason is purely symbolic since they don't use chassis codes as a marketing tool to begin with.
Also, let me restate it another way for emphasis:
No one knows why the new M5 and M6 retain the chassis code from the series model while the new M3 and M4 are rumored not to. That is, no one knows what the significance of these two diverging naming schemes is yet.