What does the X4 mean in an PCIe Gen X4 NVME drive?

PCIe gen 4 drive in a gen 3 slot? can you put a gen 4 ssd in a gen 3 slot But ya You will be limited to speeds until you put it in a matching slot If devices didn't work in slots there would be a LOT candy blitz slot If you can put up with the slightly nutty presenter, someone on Youtube did a video comparing these two drives on PCIe

can you use a pcie 40 nvme in a 30 slot First of all, I know that a PCIe 4 SSD NVMe will be running at gen 3 speeds and bottlenecked by the PCIe 3 slot But I'm wondering if there are