You've already had 4 concussions and your only 13? I would start being extremely careful with your head if I were you. I'm 17 and have had 2 concussions in my life so far, one minor and one I was hospitalized for and still don't remember what happened.
If I were you, I would wear my helmet all the time around horses, whether your on them or not.. You only have one head, one brain. If you wreck that now that's your life done and dusted, no more. Once you have brain damage that's it, you can't reverse the concussions you've already had, but you can do whatever possible to not gain more to your record.