First post for me ....
I have a Thule 3-bike carrier with a 4th bike extension and it just clamps onto the tow bar ball without any additional support.
As long as you turn the adjustment screw on the carrier to a pretty tight setting the whole thing just clamps so firmly that there's no lateral movement at all - even big speed bumps or tight corners don't upset things.
I must say that I was quite nervous during the first few journeys, with my eyes in the rear view mirror at least as much as straight ahead but if clamped firmly there's nothing to worry about.