However, some 400-series motherboards have what it takes to deliver PCI Express 4.0 support and now Biostar is the second motherboard maker to defy AMD by pushing out BIOS versions that enable PCI Express 4.0 on some of its 400-series motherboards:
The updated BIOS lets you use PCI-Express gen 4.0 graphics cards on the topmost PCI-Express x16 slot, and the M.2 NVMe slot that's directly wired to the AM4 SoC. The expansion slots that are wired to the chipset are still restricted to PCIe gen 2.0. You will need a 3rd generation Ryzen "Matisse" processor for PCI-Express gen 4.0.The BIOS with PCI Express 4.0 support is available for the AB45C-M4S (B450MH), the AB35G-M4S (B45M2), the AX47A-A4T (X470GT8), and the AX47A-I4S (X470GTN).
Via: TPU