Hasn't this thread derived?
Choice between M3 and M4 depends on looks, some think Sedan looks Better, others think Coupé looks better, and it also depends on practicality. Are 4 doors more practical for you? That is all. Maybe performance might be different, but that is a minimal difference that only plays role if you are racing an laptiming. I doubt this is of for importance for the decision between M3 and M4. The only thing is that you will (has yet to be confirmed 100%) get full-LED lights on the M4, while on the M3 you'll have to wait for facelift, of course only in the case F80 and F82 front are different. But I think it would be weird if they gave both M3 and M4 the same face while different name. So I guess there'll be a difference.