Why 4 buttons?
You must really like Mouse Trap, because it is the only 4-way vertical game that uses more than 2 buttons.
It would probably make more sense to go with the uncluttered 2 button setup rather than installing 2 extra buttons for a game you are never going to play. Even if you do want to play it, you still can simply by mapping the start buttons as buttons 3 and 4 (buttons 2 through 4 hardly ever get used in Mouse Trap).