There is no "right" length of a personal statement, however, I've heard from multiple admissions counselors that if you're given 4500 characters, use 4500 characters. At 3500 characters, that's at least 100 more words you can use to tell your story and sell yourself.
Yes I would say to use the whole length given, I was writing my personal statement and I thought 4500 characters was not enough at all. Just to confirm the 4500 character limit, includes spaces and commons?
Mine was around 4000, you really don't need to use all of the characters if you feel like you've said all you need to and have an overall strong statement! Have lots of people read it and see if they think there's anything else you could add or if it seems too short when reading it. Otherwise, I think you'll be fine!