I don't think slim builds lend themselves well to 4 player setups since the biggest 4:3 screen you are going to get is 21.3 inches, and that is decent, but not huge. The problems you will have is this:
A 21.3 inch screen is about 17 inches wide, A good 4 player panel should be at least 36 inches wide, in my opinion, so the CP width is going to be about 20 inches wider than the screen, which means that either your CP overhangs a lot on either side, making it look way too huge for the cabinet, or you make the cabinet wider, resulting in a huge bezel around the screen. Huge bezels around the screen is probably the dominant theme in crapmame projects.
I will suggest to you what I suggest to everyone else. Get either a blue tooth receiver or a xbox 360 dongle for your PC and get 2 wireless Tekken 6 joysticks. That way, when you want to play 4 players, you just turn on the sticks and people can sit on stools with the sticks on their laps, they are good sized sticks, about the size of a college textbook. You can find the sticks for 40 bucks or so apiece on ebay. It gives you a wireless solution that is pretty aesthetically pleasing. Or you can just opt for wired hori ex2 sticks, you can find them for around 35 bucks, and just plug them into a usb port.
Just my 2 cents.