i too had a 2 player cabinet with u360s and wanted a 4 player cabinet...so here is what i did:
converted the 2 player cabinet to a classics cabinet, with a vertical monitor, a single Sanwa JLW in 4way mode, 3 buttons and a trackball (centipede and millipede)
picked up an empty showcase cabinet and set it up as my 4 player cabinet. using Sanwa JLWs (in 8way mode) 6 buttons (street fighter layout) for players 1 and 2, and four buttons each for players 3 and 4. i have plenty of room for a trackball if i choose to add one. I'm not a golden tee fan, but the kids like world class bowling, so i may add one eventually. i'm just not sure if i want the added clutter on the panel.
i originally had my two u360s for players 1 and 2, and JLWs for players 3 and 4, however i liked the JLWs so much, i moved them over to slots 1 and 2. I installed some x-arcade sticks for players 3 and 4 (i had them lying around) and plan on replacing them with two more JLWs as soon as i get the OK from my wife. (promised i wouldn't spend any more money on it until we bought the kids a new swing set).
the u360s are great because of how versatile they are, but since i moved all of the 4 way games to the other cabinet, the only reason i wanted them in the 4 player cabinet was for qbert....as it turns out, the square restrictor on the JLW makes it a great stick for q-bert (just remap the game to use two inputs per direction)
i am not aware of any 4 player games that require more than 4 buttons, alot of them only use 2.