Although recent BIOS updates for X370- and X470-based mainboards were supposed to include PCIe Gen 4 support, AMD's Robert Hallock has just confirmed on Reddit that this option will not be available in the final firmware releases for 3rd generation Ryzen (AGESA 1000+). Obviously, most of those who replied to his message consider this a mistake.
https://www.notebookcheck.net/Forget-about-those-BIOS-updates-Pre-X570-mainboards-will-not-support-PCIe-Gen-4.422936.0.html