PCIe x4 vs x16 for single NVMe SSD speed increase?

If you're looking to add a PCI ExpressNVMe drive to a desktop with an older motherboard that lacks slots, one option is an drive on a carrier card

2 drives as any other PCI-e lane They will split it based on what slots are populated I know for something like Ryzen it has a dedicated x4

m2 nvme slot The Bottom Slot M2P_32G: Shares bandwidth with PCIe X4, which lowers that PCIe to X2 Currently the x4 slot is unoccupied The manual describes

m24 slot wallet Most of those cards are x16 and their switches split to x4x4x4x4 But there are x8 cards with 4 sockets which then split to x2x2x2x2 Check