AMD’s Senior Technical Marketing Manager Robert Hallock has clarified the PCI Express 4.0 situation for chipsets prior to the new X570: there isn’t one. In a comment posted in the r/AMD thread about the SweClockers report that AMD had cut PCI Express 4.0 support for 400 and 300 series motherboards, Hallock replied to a question about PCIe4 support from X470 boards:

“This is an error we are correcting. Pre-X570 boards will not support PCIe Gen 4. There’s no guarantee that older motherboards can reliably run the more stringent signaling requirements of Gen4, and we simply cannot have a mix of “yes, no, maybe” in the market for all the older motherboards. The potential for confusion is too high.

When final BIOSes are released for 3rd Gen Ryzen (AGESA 1000+), Gen4 will not be an option anymore. We wish we could’ve enabled this backwards, but the risk is too great.”