TDP is what you can expect on the average but there is no way you'll run a rx 570 when limited to just 120W per card. You can expect the cards power usage to fluctuate +-50% easily in short <1s bursts and as soon as all the cards hit their peak power draw at the same time your system will crash.
From my experience, a modded 570 with adjusted voltages and mem straps will need ~165W available per card to mine eth at full speed and stability. 750W will run 4.
Also the gold/platinum/whatever PSU's are really pointless given that the investment would have a ROI of over 5 years to match a lesser rated PSU. That's worse than buying solar panels or a small wind turbine to power your rigs in terms of profit.