At its CES keynote, AMD reiterated its guarantee of support for the Socket AM4 motherboards until 2020, so the new Ryzen processors will be backward compatible with the existing motherboards, but with a caveat: AMD says you will lose support for PCIe 4.0 on its older platforms.
But after speaking with several motherboard vendors here at CES 2019, we've learned that many of them have successfully tested PCIe 4.0 on 300- and 400-series AMD motherboards, meaning that the feature could be enabled with a simple BIOS update, at least partially.
Our sources tell us that after unlocking the feature via a BIOS update, the older motherboards supply a PCIe 4.0 x16 connection to the first slot on the motherboard, but the remainder of the slots revert to PCIe 3.0 signaling rates. That's because any trace routing on the motherboard that exceeds six inches requires newer redrivers and retimers that support PCIe 4.0's faster signaling rates. That means the PCIe slot nearest to the CPU will easily support PCIe 4.0, while the other slots, including M.2 ports, will run at a PCIe 3.0 signaling rate.