If I had to choose between M4 and M2C it would be a hard choice but with an ownership of M4, it doesn't seem to worth spending money. M2C must be a great car with better handling due to smaller size, perhaps would need a bucket seat installed in order to be driven on track though (if it comes with the same not-that-great seat as base M2). But 1) BMW deliberately made that car similar but not faster than F8x. 2) I am sure even M2CS will be slower than M4CS for same reason. 3) M2 can't fit 305 rear tires that helps exiting corner a lot
p.s. also with a base M4, it doesn't make sense to me to spend extra money to trade in with M4 ZCP or M4CS either. M4GTS..maybe...