Seems it’s more complicated than that. I had problems with my 4,1 after upgrading the Bootrom with 10.13.5. I could boot a finished installed macOS just fine, but I can’t do system Upgrades. Also I couldn’t boot into a recovery or a macOS installer stick. All stucked with a kernel panic.
when I used a second Mac to install Mojave (or any other OS) on an external disk and then put the disk into my Mac Pro then it boots und worked fine. But nothing more.
Later I realized that there was an Intel Q1DV with C0 stepping installed (engineer sample of the W3570). I always thought it were a normal W3570. So I changed the CPU and everything’s works and I can boot everything.
It’s strange for me the Q1DV could boot a installed System, but couldn’t boot up the recovery nor a Installer or perform an update.
The explanation for this behaviour is simple.
Apple implemented a way to load/inject the current available microcodes from macOS during the macOS boot process ( see /usr/libexec/ucupdate -q ), but this don't work from Recovery or macOS installer.
This was developed so we can have very early new microcode support from Intel without the need of BootROM upgrades, that takes a lot of time to be fully tested by Apple before sending it upgrade or brick millions of Macs.
That's why you can load a fully installed/updated with ucupdate macOS disk, but your Mac Pro can't boot Recovery, do updates or run a createinstallmedia macOS installer with a non-supported anymore Xeon/i7.