I have 5 x 480 undervolted rig with 1000W PSU, and with zec it takes 520W from the wall. So I thing 600 - 750W is enough for 4 x 470.
That is right. If you undervolt, 600W is more than enough.
So I guess in case of undervolting, the Sol/s is decreasing, right?
no not really i thought the same thing will lower heat and use less power but less sol maybe not if done right.undervolting doesn't all ways mean it will lower the speeds .