Piece of cake, I built one in 4 minutes.
Make a USB to XBox cable with electrical tape or solder.
Then, get a usb hub and the cable to connect it to your usb port(ooooh...).
Next, stay with me here, connect 4 XBox controllers to the hub with the cable(s) you made.
Then, connect the hub to one of the usb ports in the 360.
Good luck, and tell me how it turns out!