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

How to Add NVMe SSD to your Motherboard nvme in pcie slot Add up to four high-performance NVMe SSDs to a system with a single adapter in a physical x16 PCIe slot The system requires the PCIe bifurcation (lane nvme and m2 slot difference If you're someone in need of a clever PCIe x16 adapter to add more quick storage into your system easily, this PCIe x16 Enclosure

nvme in pcie slot Two NVMe SSD slots on a Low-profile PCIe Card Add Your Own SSDs Add up to 16TB of SSD Storage! Super High-speed Data Transfers