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

I'm not really up to speed with the M2 NVME SSD slots thing I remember the good old days when you just filled drive bays

Can I use an M2 SATA in an NVMe slot A slot that can do SATA will have the M key andor B

m2 slot screw While the traditional SATA SSDs can leverage both the ” form factor as well as the slot on the motherboard itself, the NVMe drives are

m2_1 slot We're constantly conducting extensive testing to identify the top NVMe SSDs for PC gaming While a 512GB drive may seem tempting for the money,