Gigabyte has released new AGESA microcode updates for its X470 and B450 offerings that seem to enable PCIe Gen4 support in the BIOS. The feature, originally thought to be available only on upcoming AMD X570 motherboards, can enable Gen4 speeds on the nearest PCIe slot connected to the Ryzen 3000 CPU. Other PCIe lanes going to the chipset and M.2 slots will continue to operate at PCIe Gen3 speeds.
https://www.notebookcheck.net/Gigabyte-s-latest-BIOS-update-enables-PCIe-4-0-support-on-its-AMD-X470-and-B450-motherboards.420880.0.html