Trying to write a modern CI4 project with some of the bells-and-whistles can be hard. Just found out that if you want to have a piece of the Config-magic you need to pay attention to the namespace of your files!
Using PHPstorm's New PHP Class function to create a new file in app/Config will give you a correctly namespaced class in App\Config. When trying to set values in .env and accessing the configuration using config() you will get an error:
Fatal error: Cannot declare class App\Config\BackendJwt, because the name is already in use in /Users/.../app/Config/BackendJwt.php on line 0
Say what?
After much debugging and finally reading the CI4 Documentation I noticed that the namespace should be plain Config!
Problem solved!