Have you looked at the layouts over at slagcoin? You just find the one you want, print it out full scale, and viola, layout template.
It seems to me that 29 inches is a little cramped for all of that stuff, my CP is 33" wide and it feels about right for 2 grown men, It seems to me that you may have room for a trackball, or a 4 way, but not really for 2 four ways and 2 trackballs, but to each his own. I suggest you print the layouts out and mock up a lifesize board to see how usable the layout is.
As for additional buttons for the trackball, you don't need them, just use player one buttons 4 5 and 6 for left, center and right mouse clicks.
The buttons you would need, other than controls, would be start, and exit to get you back to the FE. Some people also add pause, and you may possibly want coin buttons too. That's it.
If players three and four are going to be off the gamepad, I would just have a button on the gamepad be start.
Try to keep your CP as simple and clutter free as possible, it will save you headaches in the long run if you plan on others playing your cab. You want it to be idiot proof.