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

x4 slot   Dan x4d slot

* The PCIEX4 slot shares bandwidth with the M2P connector The PCIEX4 slot operates at up to x2 mode when a PCIe SSD is installed in the M2P

Should work fine, it's the same as putting a 16x card in to a slot that's only wired for 8x or 4x, you don't get the full bandwidth but it will PCIe boards can fit into slots designed for their lane configuration or higher Plugging a x4 PCIe into a x16 slot is acceptable

papaya pop slot x4 slots are designed to replace the PCI slot, and x8 and x16 are designed to replace the AGP and PCI-X slots Table 3-2 compares the performance of PCI 6-layer PCB made by 2oz thickened copper; Lightning Fast Game experience: PCIe slots, Lightning Gen 4 x4 with Shield Frozr; LAN with Wi-Fi 6E