So the story goes like this, Comet Lake, the pending to be released Intel desktop processors up-top 10 cores, originally had a PCIe Gen4 design, but with signal validation issues, Intel is sticking to PCIe Gen 3.0 for that release in the upcoming months. 

Succeeding Comet Lake will be Rocket Lake, and a new slide just leaked out showing it will support PCI-Express Gen4. With Rocket lake intel would be going back to 8-cores (that is a rumor though). The generation succeeding Rocket lake then would go upwards to 16 cores (8 large and 8 small cores, you know BIG-little.) back to Rocket lake though, it would be based on a ... 14nm node and the LGA 1200 socket.

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 the presence of Jitter in the Z490 chipset signal, that causes instability in the data signal and that means 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.

Comet Lake is still an x4 DMI interlink (the link between the processor and chipset), Rocket Lake would likely get an x8 connection. In addition, the processor has received four additional PCIe lanes for a total of 20 from the CPU itself. The slide also mentions Thunderbolt 4, which would provide the same speeds as Thunderbolt 3, we're not quite sure what there would be new on that front though. The slide and table courtesy goes out to videocardz.

Intel Comet Lake successor Rocket Lake to get PCIe Gen 4.0 Support