I'm not sure what you mean. It should be straightforward to connect a thunderbolt bridge to any version of PCIe.
If you specifically mean "without going over 4 lanes", then that's probably true.
If you specifically mean "without going over 4 lanes", then that's probably true.
A theoretical Thunderbolt 4 that's twice as fast could easily be fed with a PCIe 3 connection. It could even be fed with a PCIe 2 connection!
I had always thought Thunderbolt was essentially "external PCIe".