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.
Oh I did not know about 5V dummy load. What kind of load do you suggest to put on 5V?
Google found this:
https://pcbsmoke.wordpress.com/2012/11/21/converting-an-atx-power-supply-into-a-dc-bench-supply/"Q. Which power rail do I install the dummy load resistor on?
A. As a general rule, you install the dummy load on either the +5 volt rail or the +12 volt rail, whichever one has the most power."
Modern Power supply have most Power on 12 V and since we already connect GPU and risers maybe we don't need any dummy load?