After putting it off for four years, I got an HIV test a few days ago and am terrified. I had unprotected sex with an ex boyfriend 4 years ago. He had gotten tested before we started dating and tested negative, but I"m owrried that the time window wasn't long enough. He had unprotected sex with one of my friends who had had sex with a bisexual 18 year old. I'mnot sure if it was over 3 months or not.

I also used to drink a lot and I'm terrified that I slept with someone without knowing it at some point (although I think that's highly unlikely).

Am I really low risk?

yes, you are really at low risk.if your partner was positive the odds are at like 200 to 1 for you to get infected and if he was neg its impossible for it to happen.you are doing the best thing,believe in that test when you get it because it is very accurate.
good luck