What is the difference between PCIe Gen 3 and PCIe Gen 4?

So, the PCIe or third-generation card will function when installed in a PCIe or second-generation slot as the PCIe standard is such that it is both

2 PCIe 3 and 4 drives look identical from the outside The difference is the standard in the way they connect to the motherboard, and thus, the

nvme slot vs m2 NVMe Drives can use 4 PCIe lanes, which means a theoretical maximum speed of Gbps Irrespective of the form factor, NVMe

nvme pcie slot NVMe over PCIe Fabrics · High Speed Reflective Memory · Transparent Host Target IBP-G4X16-3 Backplane PCIe Gen 4 Expansion 3 slot backplane 3 Slot x16