Tom's Hardware has an interesting story about Intel's plans for PCI Express 4.0. Apparently, the chip giant initially planned to support PCI Express 4.0 with its Comet Lake platform but decided to cancel it due to signal integrity issues. Most motherboards with the new LGA1200 will reportedly have various components to support PCI Express 4.0 because this feature got cut too late during the design process. There is some hope that Rocket Lake will enable these boards to offer PCI Express 4.0 support, but that isn't sure as Intel isn't know for allowing full backward compatibility with previous-gen chipsets.
The PCIe 4.0 interface comes with twice the bandwidth of PCIe 3.0, but that also comes along with tighter signal integrity requirements. Unfortunately, Intel reportedly ran into issues with the chipset and untenable amounts of jitter (we're told the Comet Lake processors themselves are fine), thus requiring cost-adding external clock generators to bring the interface into compliance. In either case, the issues reportedly led Intel to cancel PCIe 4.0 support on the Comet Lake platform.
Comet Lake is expected to be introduced around mid-April. The platform reportedly got delayed due to the issues with PCI Express as well as unidentified challenges.
Rocket Lake in 2021 may be the first Intel consumer chip with PCI Express 4.0 support.