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

PCI Express Gen4 - 3 slot passive backplane m 2 gen 4 in gen 3 slot 2 slots will support both M Readwrite speeds of the top NVMe SSDs on the market can exceed 3000 MBs , with some Gen 4 NVMe PCIe m2 sata slot Solved: Does installing gen4 NVMe SSD on PCIe 3 slot affect the speed? or should i buy 3 gen3 NVMe ssd? Please give me some advice

m24 slot wallet 3 I225-LM x 1); 22802242 M-Key x 2, 30423052 B-Key x 1 + Micro SIM Slot x 1, Mini PCIe x 1 + Micro SIM Slot x 1; USB Gen 2 x 2, USB Gen 1 x 4