Im using 2x 750w platinum for 6x rx470 and its fine.
Im measured consumption with cryptonight mining and it only using 660W and with ethhash 970W.
Maybe i can drop consumption for ethhash by adjusting something in the biosmod.
Thats good to know so I should be able to run x4 cards with the 750w untill i get the other two cards and then use 2x750w
Just run 3 cards with 1x 750 till your second psu arrive.
It will put more stress if you use 1x750 for 4 cards and mob+cpu and will decrease the lifetime of the psu.