OK I’ve actually figured it out. This was a post created before I added the new gallery, and I still had to go into the gallery settings in the post (not the File Gallery, but the default gallery settings when you click the “upload image” button and that new window pops up), and chang the number of columns there. Now I just need to fix the borders, but I already know how to do that!
So, maybe this will help someone in the future 🙂
Thanks, alchymyth, unfortunately, that doesn’t work as intended. It only goes up to 3 wide, regardless of what column number I choose. I edited my original post to be more clear on that fact.