one of my system total power draw is around 680W with 4 RX 480
System Configuration:
Motherboard: GigaByte GA-X99M-Gaming 5 (rev. 1.0)
CPU: Intel Xeon E5-2666 v3 ES ( i know its a little bit overkill
)
Mem: 16GB VIPER
PSU: Corsair AX860i
VGA: 4x Shapphire Nitro+ 8GB with my own modded bios
System at max hashing speed for ETH (122.5MH/s) pulling 680watts/120volts at the wall
No monitor attached to the system , i set up as a headless computer , used a Display Emulator , controlling the pc with remote desktop , TightVNC , or teamviewer
Your system is will be drawing to much amps for the 4 cards RX480 setup from a 750 watts psu. it may be run fine for a short time. Your psu will work at 95-100%
to stay on the safe side get a bigger psu , or get a custom low powered rom for your cards , then u will be ok