if you have a Mac Pro 4.1 you have to verify some things in it
It is possible to use an apple firmware to flash it to latest 144.0.0.0.0 boot rom and use also OSX like sonoma (using OCLP)
Have you any OS yet installed?
if so verify here:
this is from my hack (later i will post the same thing i have in my mac pro 4.1/5.1
It is a simple procedure to do, a bit tricky but it is possible alo with a poor experience in this field (as me i mean)
Then, ye it is possible to boot and also see boot menu using your RX580, and it is also possible to flash the firmware with it
for more detailed instructions check this:
https://blog.greggant.com/post…ro-firmware-if-stuck.html
EDIT
Mac Pro 4.1 data
this below are the value that OCLP spoof/inject:
as you can see i have also update the xeon to a 6 cores xeons (Mac Pro 5.1)
this original data after installing Mojave 10.14.4
installing latest Mojave will upgrade the firmware to 144.0.0.0.0
from there you can update till latest macOS Sonoma using OCLP
i stopped my system to Monterey because i have installed a RX 6600 GPU whic OCLP supprts by now only in Monterey
your RX 580 works also in Sonoma