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

Mac StudioMac mini with M chip have enough space on mother board internally for nvme m2 slot most 2TB nvme ssd only cause 100-150 usd and

NVMe: 1 Slot · SATA SSD: 1 Slot · Micro TF: Supports 32GB, 64GB, 128GB, 256GB

slot online svizzera ASUSTOR FLASHSTOR 6 - 6x NVMe slots-image ASUSTOR FLASHSTOR 6

slot wallet ฝากไม่มีขั้นต่ํา 2280 NVMe SSD slot x1 228022110 NVMe SSD slot x1 228022110 NVMe SSD slot x1 Memory