Try meditation, reading and rest. That's what helped me with brain fog.
I am about to join a tournament in my city and I have a brain fog I only have 4 days left

Yeah, just leave chess alone til the morning ur playing. Ur not gonna really improve in a few days. Do a few tactics puzzles on a real board the morning ur playing. Get a lot of exercise & drink plenty of water, no sugar. Sleep a lot the night before. That's it.

Basing this comment on what you have said. The rest of the players will be worring about playing you, they'll have brain fog too.

I'm so nervous and excited but the problem is I can't even focus properly, this has been going on for almost a week now my accuracy is so low it reached 50% I usually get 75% accuracy, I can't trust myself with this one, everyone is expecting me to win because I achieved alot of chess trophies this past months but I don't think I'm ready. Any advice?