My father left me when I was 4. I am 15 now. He wants to try and get custody of me. But my grandparents have custody. Is it lawful for him to be able to get custody? Do I have a say so at all?
A few months ago he wrote me a letter that scared me half to death. I don't want to live with him, but do I have the choice?

It is lawful for him to try. His chances of success depend on the circumstances under which your grandparents got custody, and any extenuating circumstances such a violent history. No. You cannot choose where you want to live. A judge may consider your preference but will make a decision based on the facts, and your best interest.

I will assume you know that the grandparents went to court and have legal custody though the courts,

You have the right to testify in court where you would prefer to live, he has the right to sue in court and try to get custody