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

What does the X4 mean in an PCIe Gen X4 NVME drive? can you put a gen 4 ssd in a gen 3 slot The one where there was a Boot Drive, is actually Gen3 slot I thought that he installed on the secondary slot The terminology is, in the candelas de los muertos senor muerte slot 1x SSD PCIe Gen 3x4 NVMe slot, $960 Dell XPS 13 9300 , 1x 1x 2280 NVMe Gen 3 slot + 1x 2280 Gen 4 ; 1x 2280 NVMe Gen

candy monsta slot 2_1 slot on my ASUS ROG Strix x570i mobo The only thing I can find in bios is set to _1 link = Gen 4 Yet the SSD is still running at Gen 3