So, you didn't have enough connector for powering the 4th GPU and you plan to add one more PSU to powering the 4th GPU or maybe you will powering two or more GPU with 2nd PSU, my assumption since you didn't say how much power your 1st and 2nd PSU. It's okay with that scheme, but always calculate the power consumptions of your device.
(dual PSU cables shouldn't wire all the connections from both 20/24-pin main connectors; it should only have all the cables for one connector, then the other should only have the green pin and one ground connected).
If we are trying to do jump-start, make sure give dummy load on 5v.