It gets down to the differential tracks connecting the PCIe lanes. Tollerance for PCIe3 compared to 4.0 may see tracks on a motherboard tollerant for 3.0 but f

It gets down to the differential tracks connecting the PCIe lanes. Tollerance for PCIe3 compared to 4.0 may see tracks on a motherboard tollerant for 3.0 but fall short for 4.0. Higher the speed, the smallest difference in the length of the track routing pairs can see what works become less stable. Which would explain why some can do 4.0 and others not as well.