Dear Brightside -
First of all, congratulations on an impressive GMAT score! With a score like that and a strong application, you should be able to mitigate any negative impact of your AWA. This is a situation that might benefit from an optional essay (normally I do not suggest writing these unless you have unusual circumstances but explaining your 4.5 could be an unusual circumstance!). Keep in mind that admissions committees know that you can get "help" on your essays so just writing well may not be enough. I'd suggest that you make sure you indicate on your resume any experiences that you have conducting business in English and that you have your recommenders speak to this as a strength. Lastly, with your scores, most schools will extend you an invitation to interview. This will be an ideal opportunity to demonstrate your command of the English language.
Good luck!