Hey 👋
Unfortunately, it looks like you have leaf septoria. It also fits the pattern that only one out of four plants is affected. In my opinion, you should remove the lady, or at least isolate her and treat her with neem oil. If you don't, the fungus can spread to your other plants and also contaminate the tent for future grows.
I'll drop a link from Dutch Passion below, where you can read a lot more in detail.
https://dutch-passion.com/de/blog/blattseptoria-vs-kalziummangel-n1109
I hope it helps you. Good luck with the rest and happy growing 👊🍀