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I wouldn't be so sure. I think they can squeeze 2 TH/s out of 1100-1200W.
2000G * 0.7W/GH = 1400W MINIMUM...OK... 
The assumption is that the S4 works with underclocked/undervolted ASICS to reach a better W/GH ratio, I would assume somewhere between 0.55 and 0.65 W/GH, so one 1300/1400W power supply could indeed do. But of course we'll not know for sure till @BITMAIN reveals the exact specs.