PCIe NVMe Gen3 vs Gen4

Maybe, but only in sequential data transfers I would avoid the first generation of PCIe , as they rely on a modified PCIe controller

Yes, PCIe is both forwards and backwards compatible For example, this gen 4 NVMe SSD will work in a gen 3 PCIe slot, and a gen 3 NVMe SSD would also work

williams slot machine Eight NVMe SSD slots on a PCIe x16 card Silent operation Add your own ultra-fast SSDs for Mac, Windows, and Linux

will gen 4 nvme work in gen 3 slot slot connecting to Gen 4 NVMe drives If there is a problem, you can always change the BIOS to manually set PCIe4,3