As far as i know, there just arent any 4 player games that require more than 4 buttons..that is why it is done that way.
if you are looking to save money, happ competition sticks are pretty good. they have a decent feel to them, come in lots of colors, and don't cost much. we have 4 happ competitions in the 4player panel here at work, and nobody has ever complained about them. we also have a J-Stik set to 4way mode for the classics.
the u360s from ultimarc are completely awesome, but very expensive. they are great if you have a small control panel and want a very versatile joystick. if you are planning on a 4 player panel anyway, you have some space to work with. so there is usually plenty of room to add a dedicated 4 way joystick for the classics like Donkey Kong and pacman. (the ultimarc J-Stik/Sanwa JLW makes an excellent dedicated 4way stick). i have a 2 player cabinet at home, with a relatively small control panel. i didnt have room for a dedicated 4 way stick, so i went with the u360s. they work very well for just about any game, and they are nice because there is never any configuration that needs done, and nobody has to guess which joystick to use. however if i were building a 4 player cabinet with a big panel, i'd definitely have a dedicated 4 way stick .
i could be wrong, but i believe a TMNT cabinet used a 25" monitor.