If I've understood him correctly, Kaap believed that the CPU was the bottleneck at 4K with 4 cards, hence the diminishing returns / poor scaling with 4 cards. AMD traditionally do quite well with multi-card scaling.
Scaling tends to drop off after you get to three cards but it is no big deal, 112fps @4K is enough for current monitors.
What I would be interested in is the minimums using Mantle with 4 cards as it looks like the CPU is really holding things up @4K.
Indeed in more graphically demanding games like Crysis 3 or BF4 quadfire scales a lot better. Unfortunately here we are dealing what is basically a 5 year old game that comes with a massive CPU bottleneck. Watching it run is very much like using Civ5 (which I play a lot) with the same min avg and max fps.