TRX40D8-2N2T issues with 4 GPUs

It could be a number of things, AFAIK D4/D5 POST codes are where you should be
getting stuck if it's resources. A code only indicates an issue if it's stuck and
the system has stopped responding/progressing through POST. A loop is trickier to
figure out, cycling codes mean nothing, only the last one before the cycle repeats.
So if it's cycling df, 36, 03, 46, de, ad then AD would be the one that triggered
the loop. The problem is, AD isn't a code with any meaning, AA usually flashes
when the OS begins to load and ASRock's DrDebug code list makes no mention of
anything past AA (AB, AC etc).

It could be the riser cable, it may be having issues pushing enough power to the
GPU. You will also want to check your UEFI for PCIe slot options, there may be
a menu there that allows you to select how the lanes are split up. If it's set
for 3 occupied slots and allocating all lanes already that could be the issue.

I have only ever tried getting 3 GPUs going on my 2950x so I am afraid you are in
uncharted waters where it comes to my experience. On top of that you are using
a server grade (ASRock Rack) board so the BIOS will be wildly different.

Hopefully something I mentioned puts you on the right track, failing that you can
open a support ticket with ASRock Rack:
https://event.asrockrack.com/tsd.asp