2021.4.18.x is not the FSD Beta branch, it's the general release branch. I don't know what's in 4.22, but it might be the one for the new Model S.
The 4 is the week number that the branch was created, and the numbers following it are major/minor version numbers within that branch. There's also a 2021.12 branch out there -- maybe that's the FSD Beta branch, but I don't know.