I feel like there are already a lot of posts on these individually But I'd like to compile what we really don't know.
What is inside the FNAF 4 box?
After completing Nightmare/Night 7, the player will be taken to a grey box with locks on it. The player can click on the locks, but the box cannot be opened in any possible way. Then, a message appears, saying "Perhaps some things are best left forgotten, for now". Scott Cawthon was intentionally vague and cryptic about the meaning behind the box; it's contents and if it will ever be opened remain unknown.
2. What is the Crying Child's name?
On February 2, 2021, a Reddit user named u/Wolfie1740Kingdom made a post on the official Five Nights at Freddy's subreddit in which they possibly figured out Crying Child's first name, it being "Evan", using the Five Nights at Freddy's: Survival Logbook.[1] This is supported by the Fazbear Frights story The Real Jake, where the brother of a man named Michael, who is believed by some to represent Michael Afton, is also named Evan. As it is theorized Michael Afton is the Crying Child's brother, the other son would be named Evan.
u/Wolfie1740Kingdom themself doubts the likeliness of the Evan code nowadays. On November 28, 2021, another attempt to find the Crying Child's name using the Survival Logbook was made by a Reddit user named u/EcstaticMarzipan7 who made a post on the official Game Theorists subreddit in which they proposed that "Dave" is the Crying Child's name.
Personally I think that it is neither but close as both share and e,v,and a.
3. Foxy Grid
The foxy grid is an unknown code in the FNAF Security Logbook. The meaning of it is unknown. That basically all we got. Even MatPat couldn't crack this one.
4. Tally Marks
Tally Marks in groups of 2, 4, and 5 appear in both FNAF Security breach and help wanted.
5. Mimic
Most likely we will get more info after the release of Secrets of the Mimic. However, currently the origins of the Mimic is unknown with it possibly dating back to ennard, or possibly even the 1970's as henry's creation.
What do you think? If you want to see another mystery added to the post please let me know and I will update. Stay tuned for my next post, Personal Headcanons I'm 99.999999999% sure are true!