Well let's see players that are in front of him
Federer - Will still be #1 by the end of the season
Nadal - Will be #2, will defend most of his clay court points probably will win Roland Garros and get a good showing at Wimby
Djokovic - I'll hold my opinion till after Roland Garros, but he has the potential to drop a lot of points if he doesn't fix his serve fast
Murray - Same, he could fall out of the top 5. He needs to get his head on straight
Del Potro - He won't make it to the semis at Roland Garros, and he won't defend US Open. He might get a masters on the hard courts but I doubt it, this guy has been out of it for too long maybe next year
Davydenko - Will miss a lot of tournaments due to injury, and because of that fall far in the rankings
Soderling - Will not get to finals at Roland Garros, but he can pick up a lot of points on the clay courts. Maybe pick up a hard court title or two
Roddick will at least surpass Davydenko, and most likely Del Potro. So that puts him at 6th, Soderling and Djokovic are the two I believe he could surpass to get to 4. It's not impossible, but he does have to rely on people to lose their points.
He has the possiblity to get points on the American hard courts as well as a better showing at the US Open and more hard courts after the US Open since he only played 2 tournaments and only got 10 points after the US Open