try to take a look at the market section, plenty of psu there, you need one 750w to be safe, better if it come from a good brand, don't buy cheap psu please, they can toast your miner
Seriously, S4 consumption is 1400watt, not 700 watt.
Im guessing he read it wrong on model unless he was going to use 2 atx of 700. Using 2 PSU's a pain though I did it on a original dragon 1T after it's psu stopped working, it worked fine just adds a few steps.