The possibility of PCIe 4.0 support on the upcoming AMD Ryzen 4000-series (codename Renoir) desktop APUs isn't looking too good. Looking at the product page for the Biostar Racing B550GTA motherboard, you'll see notes clearly stating that PCIe 4.0 support "depends only on Ryzen 3rd Gen Matisse" and "PCIe 4.0 speed only for AMD Ryzen 3rd Gen Matisse."
Many rumors about AMD's Renoir APUs have surfaced over recent weeks. In a nutshell, the Ryzen 4000-series could span up to eight Zen 2 CPU cores with Vega Compute Units (CUs) that scale up to to 2,100 MHz. One leak has claimed that Renoir will feature a 2,000 MHz Fabric Clock (FCLK), meaning we could see the APUs leverage DDR4 RAM modules up to 4,000 MHz.
But nothing we've heard so far has hinted at whether or not Renoir will adopt the PCIe 4.0 interface.
Although, the mobile Ryzen 4000-series chips debuting with PCIe 3.0 support may have been a clue. AMD made a rather convincing statement on why mobile Ryzen 4000-series parts retained support for PCIe 3.0. The chipmaker intentionally excluded PCIe 4.0 support in favor of power reduction. Power consumption shouldn't be a big concern on a desktop chip, but to bring PCIe 4.0 support to Renoir would require AMD to draft up an additional new design. In all likelihood, the desktop variant will use identical silicon to what's inside the mobile variants.
Similar to the previous generation of APUs, Renoir will seemingly continue to be stingy on the PCIe lanes. The Racing B550GTA's specification table says that Renoir will only provide eight PCIe lanes to the primary PCIe x16 slot. Therefore, graphics cards are limited to half the available bandwidth on the slot.
APUs target shoppers with tight budgets who aren't likely to purchase a discrete graphics card. Therefore, if Renoir does arrive without PCIe 4.0 support and with limited PCIe lanes, it shouldn't be a deal breaker for the chips' intended market.
On the other hand, an APU owner is more likely to pick up a PCIe 4.0 SSD. These SSDs aren't cheap yet, but have become a lot more accessible. So it would be nice if Renoir did debut with some sort of PCIe 4.0 support, even it's just for the M.2 slots.