If you only have one cooking grate, then you will need a rib rack and they should fit fine. I put my water pan on the bottom cooking grate in my Char Broiler Double Chef and can get four racks of bbs on the top grate alone. I'm not sure of the diameter, but it's comparable in size to the standard 55 gal drum.
I think you are out of luck getting 4 racks of ribs on each rack. UNLESS you cut them in half then you may be able to get them on there with some creative arranging but it will be close. I can fit 3 full racks on each one of my grates.