This might be old news (haven't read the whole thread) but on my MSI x570 Tomahawk (WIFI) with latest BIOS that doesn't seem to be true, I just set 1.1125V SOC and PCIe 4 is still active (as seen in GPU-z and validated via 3DMark's PCIe throughput test @ 26.5GB/s).
I was trying to stabilize my IF @ 1900 (SOC on Auto which was 1.1V) which was stable in normal use, but was causing a sudden instant reset in Prime95 (AVX @ 128k) after a few minutes of running fine. It seems to have helped, but too early to be sure.