Having built a few mame cabs, my advice is...
Too many MAME cabs try to be everything... resulting in a cab that CAN play everything, but none of it well. A giant Spruce Goose control panel with controls all over the place and in the way of each other with buttons spilled everywhere. A four-player will neccessarily have to be big.
I would run the following thoughts through:
1. Do I really want/need a 4-player? How often will it get played with 4 and is that frequency worth impeding the more frequent solo/two play?
2. Do I want controls that play everything mediocre over dedicated controls that play limited things great?
3. Here’s a brainstorm... consider some alternative 4-player designs. They don’t all four HAVE to stand shoulder to shoulder. Maybe a two-sided “head to head” design with two “normal 2-player cab” looking sides facing each other. With flat screens this is totally do-able in the same footprint as a normal 1-sided cab. Or even two seperate cabs where one cab can be switched to serve as a P3-P4 slave to the master cab.
4. Is Fruit Stripe gum white sticks of gum with colored stripes, or colored sticks of gum with white stripes, and is that tastier than Hubba Bubba?
5. Be sure and peruse Crap MAME to get some good ideas of what NOT to do.