With a 4 player panel I would put the coin buttons down lower on the cabinet by the coin door (to keep the panel clean). And then add all the start buttons at the very top in the middle. You could actually do away with them altogether if you wired up either a 4-player coin mech, or a shift button (shift the start buttons to insert coins for the respective players).
Most spinner games play allright with a trackball. I never saw the need to add a spinner to a panel that already has a trackball on it.
The panel I use is only a small one player panel. It is a Cherry Master cabinet and the control panel is about the size of a Pac-Man panel. I had someone make me an all metal panel.
I have (from left to right), 8-way leaf switch stick, 4-way stick (Ms Pac/Galaga replacement stick), 6 buttons (Street Fighter layout), PS2 Arcade trackball, then Player one and 2 start buttons, and an escape button. The start buttons are Mouse 1 and Mouse 2 respectively. The escape button is hooked up to the Ipac and puts in credits in MAME, but it is wired up as Escape, because so many other applications and emulators have escape hardwired as a needed function to exit or get to the menu.