Thank you for the suggestion! I just got done following it to the letter. Fresh Win 10 install. Installed all critical updates and guest additions before doing any PCI passthrough. Turned off the Win 10 device updates and shut down VM. Pass through 4 GPUs. One of them is connected to a display. Start up VM and everything is fine, although the little spinning dots at startup are spinning much faster than normal. Not sure what that means. I'm starting to recognize fast spinning dots with imminent crash. When I only have one GPU connected the dots spin at normal speed. More than one GPU and they spin really fast.
So I'm back at the desktop and I see the four GPUs in device manager with the yellow indicators for not having drivers. Waited a couple minutes to make sure Windows isn't installing its own driver. Then I downloaded and installed the latest Radeon driver for the RX580. During installation I watched the device manager. The screen flickers each time it installs a driver for a graphics card. After the first flicker I could see in the device manager that one card was configured and three to go. The second flicker never came back. Just hard crash.
Let me just mention that the host is fine throughout all of this. The PCI passthrough seems to be working and the host is stable. The VM just continues to crash at the second GPU. Is it possible that ESXi is mapping the second GPU incorrectly to the VM somehow? Would there be some way of checking this?