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

Factoring in overheads such as encoding latency, heat and other real world dynamics, this speed comes down slightly, to a tested Gbs

My laptop has a gen 3 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

m2 expansion slot Just to be clear, you can run a PCIe SSD in a PCIe slot , it just runs at PCIe speeds Which kind of negates the purpose

m2 sata slot NVMe SSD to PCIe x4 Mobile RackBackplane with Removable Tray for PCI Express Expansion Slot, Tool-less Installation, PCIe Hot-Swap Drive Bay, Key