EDIT 5/19/19 7:30am PT: We confirmed that a PCIe 4.0 option is available on X470, but reports have surfaced that the option is also available for the budget-oriented B450 motherboards.
Over the last few weeks, motherboard makers have released a steady stream of BIOS updates to support AMD's Ryzen 3000 processors, but Gigabyte's newest BIOS has a special feature: The company has enabled the option for PCIe 4.0 in the BIOS of its X470 Aorus Gaming Wi-Fi 7 motherboard. That signals that, under some circumstances, the company will likely support PCIe 4.0 on existing motherboards if you drop in a new Ryzen 3000 processor.
An alert redditor noticed the new setting in Gigabyte's F40 BIOS, and our motherboard team verified that this option now appears in the Gigabyte BIOS under the PCIe Slot Configuration submenu.
Our team also verified the setting isn't available with the previous F3c revision of the BIOS.
Gigabyte hasn't officially announced PCIe 4.0 support for its 300- and 400-Series motherboards, so it is unclear which models would support the faster interface, or if there will be limitations. It should go without saying, but you would also need a processor that supports PCIe 4.0 to unlock the faster transfer speeds, so current-gen Ryzen CPUs will still operate at PCIe 3.0 speeds.
AMD will also support PCIe 4.0 with its next-gen 500-series motherboards, ushering in a new wave of powerful devices like faster SSDs, but as part of AMD's standard promise of backward compatibility for AM4-Socket processors, these new 7nm processors will also drop into the existing X370, X470, B350 and B450 motherboards that are already on the market. (Support for A-series motherboards is still curiously up in the air).
We've known for some time that, under some conditions, existing PCIe 3.0 motherboards can support the faster signaling rates of the PCIe 4.0 specification, but AMD's motherboard partners have not explicitly announced that the feature would come to existing models.
At CES 2019, AMD confirmed to Tom's Hardware that 300- and 400-series AM4 motherboards could support PCIe 4.0. AMD will not lock the out feature; instead, it will be up to motherboard vendors to validate and qualify the faster standard on their motherboards on a case-by-case basis. Motherboard vendors that do support the feature will enable it through BIOS updates, but those updates will come at the discretion of the vendor. AMD also cautioned that most motherboard vendors might not bother supporting the faster interface on existing motherboards.
In either case, on existing motherboards, support could be limited to certain slots based upon board, switch, and mux layouts, and any trace routing on the motherboard that exceeds six inches would require newer redrivers and retimers that support PCIe 4.0's faster signaling rates. That means some older motherboards could supply a PCIe 4.0 x16 connection to the first slot on the motherboard, but the remainder of the slots would revert to PCIe 3.0 signaling rates. Any PCIe slots with an attached PCIe 3.0 switch would also be excluded from supporting the faster speeds, so some of the fancier motherboards with elaborate PCIe routing setups will not be eligible. All PCIe lanes that hang off the PCH will also remain at PCIe 3.0 speeds.
That means that we might see some complicated support matrixes appear to outline which previous-gen AM4 motherboards, and which slots, will support PCIe 4.0 when used in conjunction with a Ryzen 3000-Series processor.
But the larger story is that it appears support will come in some fashion, which is quite the bonus upgrade path for enthusiasts with Ryzen-powered systems. Meanwhile, we aren't sure of a firm arrival date for PCIe 4.0 for Intel's chips, and given the chipmaker's proclivity for frequently introducing new sockets, that support likely won't come to existing motherboards.
We've pinged Gigabyte for more details and will update as we learn more.