can you use pcie 4.0 in 5.0 slot
Can I Use a PCIe 4.0 Graphics Card in a PCIe 3.0 Slot? - TechReviewer
You can use a PCIe 4.0 SSD in a PCIe 5.0 slot, and it will work just fine, at PCIe 4.0 speeds. Similarly, a PCIe 5.0 SSD can be used in a PCIe 4.0 slot, but again, its performance will be limited to PCIe 4.0 levels.
Ram Issues / DIMM_A1 DIMM_A2 not working
Latest M.2 Support: Onboard PCIe 5.0 M.2 slot, two onboard PCIe 4.0 M.2 slots, and two PCIe 4.0 M.2 slots on an ROG DIMM.2 card, all with substantial cooling solutions ...
m.2 pcie slot - etek.com.vn
Next-Gen M.2 Support: One PCIe 5.0 M.2 slot and two PCIe 4.0 M.2 slots, all with heatsinks to maximize performance.