1. If you just Use Join on these data sets, doesn't it automatically join the groups of 4?
2. For lofting you need to supply lists of section curves. So if you want to loft curve {0;0;5;0}(0) in the left list with curve {0;0;5;0}(0) in the right list, you'll need to Graft both sets, then plug them both into the Loft S input.