A new report on the web indicates that the upcoming Comet Lake-S processors (tenth generation of Intel Core for desktop) actually do support PCI Express 4.0, some Z490 motherboards are designed for it as well. However an issue prevents PCIe 4.0 to become usable, so the platform will support PCI-E 3.0 instead.

The reason behind this is presence of Jitter in the Z490 chipset signal, that causes instability in the data signal and that menas that specifications required for the PCI-Express 4.0 certification are simply not met. PCI-E 4.0 is working at very high frequencies, and that requires that the signal be completely clean.

Several manufacturers added external clock generators to eliminate this Jitter, however not everyone did, so Intel made the decision to not support PCI-E 4.0 on this new platform. 

Intel Comet Lake-S CPUs and Z490 boards support PCI-E 4.0 (but it's not active)