Five Nights at Freddy's 4
Five Nights at Freddy's 4
Attachment
To build hype for the Halloween version of the game, the image on the game's website was temporarily changed to a seemingly blank image that when brightened revealed the new nightmare variation of The Puppet. The image itself was internally named, "Dontwakethebaby.jpg", this was likely a reference to popular YouTube Let's Player Markiplier, who referred to the puppet in Five Nights at Freddy's 2 as "The Baby".
Contributed by Boyobmas
Attachment
During the cutscene/minigame that occurs after completing Night 2, the player is prompted to walk left to escape a room at the pizzeria. Going right instead will allow you to catch a glimpse of a character only known as "The Purple Guy", who appeared in the previous game in the series.
Contributed by Boyobmas
Spoiler:After completing Nightmare mode in the "Extra" menu, typing "20202020" on the same menu unlocks a new mode called "20/20/20/20". It plays like the Nightmare mode, but the animatronics are even more aggressive.

Beating this mode gives a fourth star.
Contributed by Blase005
One of the ambient noises that can heard during gameplay sounds like garbled speech. When played backwards it's revealed to be a clip of Phone Guy's first call from the original Five Nights at Freddy's.
Contributed by Boyobmas
When looking back at the player's bed, there's a rare possibility that they will see one of three items that will disappear when the player looks away. These are a pot of flowers, a bottle of pills or an IV Drip. These are items most likely found beside a hospital bed.

Spoiler:It's revealed at the end of the game that the protagonist is shoved into the mouth of an animatronic which bites down. It's theorized that this is the "Bite of 87" mentioned in the first game and that the game takes place in the mind of the victim who is in a coma due to the injury. However, other clues lead people to believe that the game actually takes place in 1983, so it cannot be the infamous event of '87. It's possible that this event is actually a second bite incident that has not been mentioned in any previous games.
Contributed by Boyobmas
When the first teaser image was released, the site had "glitches" that were repeating 7's and 8's, and when fans put "8788788778" into Google Maps they found a small pizzeria. Fans were calling and harassing the business so much that Scott Cawthon had to tell everyone that they had nothing to do with the game, and edited the site to remove the 7's and 8's. The owner of the pizzeria, after finding out what had caused the confusion, offered a "Fazbears pizzeria special" for a limited time.
Contributed by gamemaster1991