How to determine from command line if NVMe slot is gen 3 or gen 4?

What does the X4 mean in an PCIe Gen X4 NVME drive? nvme in pcie slot NVMe has emerged as the dominant interface for SSDs The NVMe protocol provides an optimized command set for accessing a PCIe SSD NVMe exploits the parallelism nvme vs m2 slot difference Two NVMe SSD slots on a PCIe x8 card Silent operation Add your own ultra-fast SSDs for Mac, Windows, and Linux

nvme slot vs m2 cards with a B Key interface can only fit into a B Key host slot, and likewise for M Key But cards with a B+M Key design can fit in either