I don't think that a score of 4.5 will affect your application, particularly if you are a non-native speaker (since you'll also have to submit TOEFL scores). Remember, no one cares about the AWA in rankings, so there's honestly no point in stressing over them.

I understand that AWA scores are "there" just to make sure you have not had someone write your app essays for you (i.e. an admissions consultant who did not merely edit the essays). If you got 2 in the AWA but presented stellar essays, someone might raise an eyebrow. But I think you'll be just fine.

Even if I am applying to top 10-5 ranked schools (UC Berkeley, Northwestern) and I am a native speaker, but just had a bad awa session? Despite my low score I feel confident that my application essays will be much better written and could remove any doubt of writing deficiency.

I honestly don't think you have anything to worry about, even if you'd be applying to HBS. As I've said, the AWA is just there for the minimal check.