can i use pcie 4.0 in 3.0 slot: PUT YOUR GPU IN THE TOP PCIE SLOT IMMEDIATLY. What Is PCI Express 5.0, and Why Does It Matter for the .... Will PCIe 3.0 x4 10Gbe NIC Work in PCIe 3.0x1 Bandwidth?.
It's worth noting that the PCIe standard is backwards compatible, so you can, for instance, use a PCI-e 4.0 graphics card in a PCI-e 3.0
slot without issue, although you will see a small performance hit.
I tested PCIe 3.0 x4 10Gbe NIC in a PCIe 3.0 x1 slot, and here's my thoughts and results.