I think the roster should state the requirements. If you meet all the requirements, I don’t see why you should not be eligible to apply for that roster. Some organisations have certain time-in-post requirements before you can move. But in principle that shouldn’t prevent you from being on a roster.
The other way round should not be a problem either. If you are already on a G6 roster, I don’t see any problem applying for G4. There should not be any impact.
Good luck!