I've never done 4 racks, but I do 2 racks on 2 shelves all the time and they turn out great! So add me to the no problem camp on doing 4 (or more?) racks. The extra meat won't appreciably lengthen the smoke time, but I do like the suggestion of rotating the racks somewhere around the middle of the smoke. As for the vertical rib racks, I've never used them, but I've never seen a post on here that sings their praises. And don't worry about cooking too many ribs (is there such a thing?), they freeze beautifully, and can be reheated either in a sous vide bath, wrapped in foil in the oven, or in a slow cooker with a bit of liquid.