There are already 8K displays. They use two DisplayPort cables.
I doubt we'll see Thunderbolt 4 until quite a while after PCI Express 4.0 is released. Which will probably be next year. But there are also rumors that manufacturers may skip PCIe 4.0, since the PCIe 5.0 spec is due to be finalized right about the time PCIe 4.0 is ready to release. They may just delay upgrading from PCIe 3.0 for an additional six months and go straight to 5.0.
Thus, Thunderbolt 4 might very well be based on PCIe 5.0.
That said, DisplayPort 1.4 is available now, just not in Thunderbolt 3, and DisplayPort 1.4 is capable of running 8K displays over a single connection. The next version of DisplayPort should come out this year, allowing even higher bandwidth.