They're very similar, but there are differences. Most notably, Thunderbolt 4 guarantees a minimum speed[0] for compliance, whereas USB4 has a lower minimum.

Also, I believe USB4 was designed for smaller files / blocks and Thunderbolt is designed for more raw output.

[0] https://liliputing.com/2020/11/differences-between-thunderbo...

Thanks for the correction, and for citing such a great reference!