Honestly, you asked the question already in this thread:
http://f80.bimmerpost.com/forums/sho...1#post16031971
Possible answers won't differ I believe. Seats in M3 and M4 are about at the same position, the roof in the M3 is a little higher. Thus the M3 would fit better.
But as you were sitting in the 435i already and had enough headroom you shouldn't have anything to worry...