All those that know the M3 will know what the M4 is. All those that won't know what the M4 is, neither will know what an M3 is or was. For them it will just be a "BMW". Most funny thing is that in the mind of many "M4" already exists.
"What car was that? Z4, M3? M4???" Wether you talk to girls or even guys. They often confuse M3 and Z4 and call them M4.