I think your mobo is your problem, other users have same problem with new generation intel board (socket 1151), but it should be ok for 4 gpu. (View Post)
I have a similar rig 5 rx 480 8g and 1 r9 280x in eth mod only and with 30.5 lowpowerrom it draws out 1150 from wall (little undervolt for 280x) , you stay on boundary but dont go for dual (View Post)