I noticed when I updated the game (on the itch app, if that is relevant) that it simply added the new 3.1 version rather than replacing the old version in the files. So I had to manually go in and delete the old version.
I could be completely wrong but that might be why some people are saying that they can't access day 4? Because I only noticed it when I clicked "launch" and it gave me 2 options--both with the exact same name and file size--and I decided to actually check the files instead of just clicking one, which is how I realized both versions are there (rather than replacing the old version like I was expecting). Idk if any of this is helpful or not, sorry lol