Hmm, I noticed that each additional card would add marginally more to my draw from the wall. Not as much as you're reporting - maybe about 5W/GPU (that is 1 GPU adds 130, 2 adds 135 (265 total), 3 adds 140 (405 total), etc.). I just chalked it up to extra CPU, lowered PSU efficiency, or something. Are you saying 2 cards bumps you by 260W, but 4 cards bumps you by 640W?
370W and 720W total.