By the time we have devices that are limited by gen 4 pci-e these systems will be so outdated that you wont want to pair then with these chips.
I mean Sandy bridge was the first to use PCIe 3, imagine how much a 2500k would bottle neck with a 2080ti and that card does not even saturate the connection.
I think it would be foolish to think anyone needs gen 5 at the moment, not in the consumer space anyway and probably rarely in the professional/ scientific useage