I mean, it's not exactly a completely different car.
The suspension/steering/drivetrain is a direct carry-over from the M3/4, and the underlying 2 series chassis is simply a shortened version of the M3/4s, afaik. They weigh right on top of one another as well.
What's completely different about them? That the M4 is longer? Or that you can get nicer leather?
As far as the rolling chassis is concerned, they're actually very similar cars.
Certainly with the S55 in it, the M2 would cannibalize at least some M4 sales...and I tend to think a lot of M4 sales.
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'02 S54 M3 (500/500 GC/Koni)
'08 N54 135 (JB4, DCI, BMW PS/Bilstein B6s, H&R M3 FSB, Strongflex FCABs)
'14 N55 X1 (JB4, BMS DP, BMS Intake, Alpina TCU reflash, H&R Sports, Bilstein B6s, E93 M3 RSB, Strongflex FCABs, baby seat)
'08 N54 535xi touring (Bilstein B6s, Downpipes, MHD tune, baby seat)