It would be cheaper to buy an I-Pac than hack 4 sidewinders; however if that is really what you want to do contact me. I might still have 4 or 6 hacked sidewinders laying around that I'd sell for the price of an I-Pac

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If it were me. I'd not force the cocktail to be anything other than it is and that would be a single player vertical cabinet meant to play Pac Man's, Galaga, Galaxian, Mr. Do, Dig Dug, etc. If you really wanted to add something to it, put either a spinner or a trackball on the P2 side for a few added games (Centipede, Arkanoid, etc.). It will never be a 4-Player Midway Style cabinet, don't try to make it one.
My 2 cents.