Question: are pci express backwards compatible?
Answer: PCIe is also backward compatible. If you have a PCIe 4.0 graphics card you can use it with a motherboard designed for PCIe 3.0; however, the card's available bandwidth would be limited to the capabilities of PCIe 3.0. Conversely, a PCIe 3.0 card can fit in a PCIe 4.0 slot, but again it would be limited by PCIe 3.0.