You haven't undervolted?
900-100W from wall seems impossible for 4x 280x. I run 1100 from wall with undervolts from 1.200v to 1.143v on each card.
Depends on how far you undervolt. Some of our Gigabyte 280x Rev2 could undervolt as low as 1.01V @ 1040/1500, and our 5 GPU rig would pull 1250W at the wall. If you were to take out one of the GPUs, I bet you it would pull just a hair over 1000W, maybe 1020W?