if i'm not wrong, 4x rx 580 is running even with 750 watt psu (after mod surely).
but that will be risky perhabs. 850 watt psu maybe the best choice for this quad rx580 setup
but a thousand or more watt psu is good, if you want to add more gpu in the future
beside it's overkill, it has more cable that make the rig less cluttered
especially with gold class psu, that surely build with better quality.