I'm a 4.0 player, so here's a brief synopsis of how it works for me:
against a 3.0- 6-0, 6-0
against a 3.5- generally its in the area of 6-2, 6-2 or 6-3, 6-2
against a 4.0- it can go either way, depending on how they play. I beat a S&V player 6-3, 6-2. Lost to a baseliner 2-6, 0-6 (in defense of that score, he was really tough, one of the strongest 4.0's around). I most recently split sets with the best 4.0 in our club (my biggest rival and also good friend) 4-6, 7-5 (then we had to stop).
against a 4.5- I've only ever played one 4.5 player and lost 1-6, 0-6.
against a 5.0- again, I've only ever played one 5.0 player and lost 1-6, 1-6.
It all comes down to style of play and who can exploit weaknesses best when you play against players of your own level. Can you take them out of their comfort zone before they take you out of yours.