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

It seems like yesterday AMD introduced us to their Ryzen 3000 series of CPUs, which came with PCI Express as one of its headlining features

3 x PCI 2 x PCI 1 x PCI mini-PCIE Yes LAN 10 Gigabit 5 13, Intel X299, 8 x DDR4 DDR4 4200+, 4 x PCIe x16 1 x , AMD 3

pcie slot bracket As with previous generations, the standard simply doubles the speed that the PCIe slot runs at It now provides about 2GBs per lane

pcie x1 in x16 slot Like USB, PCIe is forward compatible This means that a device which requires PCIe will work in a PCIe or PCIe slot Slot Size Lane Width