The Xbox Series X SSD is not a PCIe 4.0, but a PCIe 3.0 x4 WD SN530 SSD
The Western Digital WD Blue SN500 SSD Review: Moving The Mainstream To NVMe
Western Digital Corporation SDBPTPZ-512G Western Digital PC SN530 SDBPTPZ-512G 512 GB Solid State Drive - M.2 2230 Internal - PCI Express NVMe (PCI Express NVMe 3.0 x4)
Xbox Series X WDSN530 has custom ASIC to support PCIe 4.0 (Tweaktown corrects PCIe 3.0 claims) | NeoGAF