Does flying actually fit with Air Bending?  I might be wrong, but the only time we actually see him fly is in the Avatar state in the series finale.  Even the flashbacks in LOK don't show him flying and he's been a fully realized Avatar for many years by that time.  Further, in flashbacks of other Avatars they are never depicted as flying.  He's still the Avatar, capable of mastering each element and controlling the Avatar state.  The Avatar is always the most powerful bender simply because they can access each element.<br/><br/>As for Azula being four years stronger, I'm not sure how much stronger she can actually get.  Same goes for Toph.  I would think there would be a pinnacle for each element and you can't get higher than a pinnacle.  Of course, there's a few ways to get around this such as creating a new type of bending or creating a new move, such as attacking with lightning or flying.  I think since bending is an art form the things that set benders apart is admittedly power, but also creativity.  I think this is illustrated the most in Azula and Zuko's final duel.