NVME drive not working in second slot of Asrock B450

What does the X4 mean in an PCIe Gen X4 NVME drive? m 2 nvme slot Storage devices using SATA Express and the specification which support NVM Express as the logical-device interface are a popular use-case for NVMe and have m2 vs nvme slot B650M-HDV · Build for The Most Advanced PC · Optimized VRM Design · DDR5 Memory with Protection Circuit · Reinforced Steel Slot

m2 pcie slot 2 slot that can run at NVMe PCIe bandwidth, whereas others may be restricted to using SATA bandwidth Some motherboards' slots may even