Salutations Viktor,
The felix-server4 branch is specifically where I push development progress for the 'leoserver.py' file in leo/core.
Indeed, most of the time it is equal to 'devel', but when I'm working on new server features, that's the branch where you can see and test those new server features, until they are judged stable and 'clean', after which I usually make a pull request for it to be merged in 'devel'.
In other words it's the dedicated 'feature' branch for leoserver.py development :)
Thanks for your interest! And don't hesitate to make suggestions, remarks or anything else!
Cheers!
Félix