I had to change my psu, because 2x20A 12V rails (400 watt) didn't work. It was fine with 1300mV setting, but quit working with 1350 mV.
A 450 watt psu with one 37A rail helped, no problems for more than 12 hours with hashing around 200GH/s.

I really would like to know how much power is needed for 4 boards.