I can log into the main game but switching to a Leagues world gives a notification for Leagues 4, is this just a typo or are the leagues worlds broken because there is a setting somewhere that hasn't been updated?

This is just a messaging typo, we'll get this fixed but it's a lower priority than other issues for the time being. Rest assured that once worlds are open, it'll be Leagues V that you're logging in to!

That's effectively from people trying to log in but ultimately not able to access those worlds, so not strictly a bug but a 'quirk' of having worlds visible but inaccessible