what about PCIe connectors?
I would need minimum 6 *8 Pin PCIe connectors from the PSU for 4 GPUs plus 4 risers: 4 of them would power 4 GPUs (150W each), 2 of them would power 4 risers, using 8 Pin Male to Dual 6 pin Female (2*150/4=75W for each riser).
Correct?
So you are planning on using Y splitter 8 to 2x6 pin, it's enough to power your riser. But your PSU only has 4 PCIe 8 pin, right? You could use a riser with a Molex connector, so you can powering your riser directly from PSU using Molex connector or else use 6 pin riser with Molex to 6 pin power. Will safer than using SATA connectors.