haha, I thought the 4th button was for the little finger
I've never used my thumb playing in the arcades, but built the above 8-button layout in cardboard and love it!
although the thumb button does feel weird, maybe 'cos I'm using concave buttons?
for 2- and 3-button games, which buttons are you meant to use? the sega layout? or the top left buttons?
Oh, and regarding the original question, I think you have to have the fourth button