Well, like I said, I can't think of any game that you would need more than 3 buttons, and that would only be for Missile Command. I'm not saying there isn't a 4-button trackball/spinner game, but if so, I don't know of it.
Here's what I would do, I'm sure others have a different (read: better) way of setting this up. I would use an IPAC for buttons 1, 2 and 3 as well as player 1 and player 2 buttons. However, you have some issues to resolve because if you're using a frontend instead of Windows, you're going to need an "ENTER" command available to select your games. Without the "SHIFT-Joystick Left" available as the ENTER command, you would have to go to non-default settings on your IPAC and program something else. Also, without a joystick, you're going to need a way to scroll through the games on your frontend's main menu.
If you don't use a frontend and just load up from Windows, you are going to need a "mouse left click button" - you can't (at least not that I'm aware of) use one of the IPAC buttons for this. Not that big of a deal I guess, just stick that button up top and out of the way somewhere.
You could set your IPAC settings so that holding down button one and button 3 at the same time equals ENTER and holding down all three at the same time would bring up the TAB menu, or any such combination that you think would work. It just seems like a lot of programming buttons. But the bottom line is that it would work, I think it would be easiest if you use WIndows. You would need three fire buttons, two player select buttons and a mouse button.