My motherboard supports PCIe 3.0. If I use an NVMe PCIe Gen4 on an X16 slot of PCIe 3.0 (by common adapters) instead of an M.2 socket, do I get the full speed of the PCIe 4.0 NVMe?
If I use an NVMe PCIe Gen4 on an X16 slot of PCIe 3.0 ... do I get the full speed of the PCIe 4.0 NVMe?
PCIe 3.0 and PCIe 4.0 are like different "languages" that are used over the same physical connection. If your PC does not "know that language" it will fall back to the lowest common version of PCIe that both devices support.
I expect that the adapter will work, but it will work with PCIe 3.0 speeds.