I just got my official GMAT score report, and I'm not too happy with my performance on the AWA. I scored 760 on the quantitative/verbal section but only a 4.0 on the AWA. I've heard that many top business schools do not put a lot of weight behind the AWA, and I've also heard that admissions officers are sometimes wary of applicants who retake the GMAT when they already have a high score.
The AWA is a gauge to ensure that the person writing the application essays is the same person who took the GMAT. They don't look for 4.0 vs 4.5... vs 5.0's.. or 6.. What they want to look for is people who score a 2.0 but submit extremely well written essays (suggesting that they may not have been the author). A 4.0 is fine.
Retaking the exam with a 760 is going to show the admissions directors that you lack an ability to grasp "the big picture".