I'm not sure you'll be able to fit 4 racks of ribs on the main cooking surface un less they're small. The upper shelves are the same length as the main cooking surface, with about half the depth.
Here's a pick of a large rack of spareribs (6.5 lbs) I cooked on the middle shelf:
View attachment 492501
There's no way I could have gotten 4 of them on the cooking surface; probably 2, maybe 3.
Also, the last few cooks I've started using a small water pan on the right side of the main surface. It seems to help even out hot spots in the cooler. However, if you chose to use one, it would suck up cook space. You might want to lean towards the 1050.