If you do 4 you have less of a load going through cables. I always have done 4 on S5. I know some will say 2 with specs could do the 590 rated at. But keep in mind bitmain recommends: "PSU: A power supply unit is not included, and you will need to provide an ATX PSU. There are 4 PCI-e connectors for +12V DC input and all four are required. Do not connect more than one PSU to the same hashing board."
So 4 is what they say are required. And granted if you underclock this could change.