Question: is pci express 4.0 backwards compatible?
Answer: Like PCIe 3.0, PCIe 4.0 is forward and backward compatible. This means that PCIe 4.0 can be used as a direct replacement for PCIe 3.0, but it also means that if you connect a PCIe 3.0 card to the PCIe 4.0 slot, the card will perform to the PCIe 3.0 specs.