Drop all but 4 buttons on players 3 and 4... no game (in mame) uses them...
Next, get some cardboard and try it out with fake components. Make sure everyone can sit next to each other and still use the controls.
Next, check to see if you can play 2 player smashtv and total carnage... GREAT 2player games...
But other then that... looks good... I personally don't like the rounded button layout (instead of two straight rows)... but thats because the streetfighter games I grew up using didn't have them... Better ergonomics sure... But I kept missing the buttons... so I switched back.