Main difference is: 4.5 is just a recreational hacker, vs 5.5 is a tennis player
Not meant as an insult since I am a 4.5 myself, but at 5.0 and below, we do not have complete games. Whereas players at 5.5+ have complete games - all the shots, footwork, and physical skills.
Why do you say this? I am a 4.0/4.5 and I think my game (and the game of many other players at this level) is fairly complete. Of course, a player who is just a little faster, a little more consistent, a bit more precise and hits a little harder would beat me every time. After all, even someone with exactly the same game as me but two steps faster getting to the ball would beat me any time. Does not mean that his game is more complete than mine.