Hey there! This is the 4th part of altering FNaF, covering FNaF 4. You probably already know how this works after 3 whole parts, so without any further ado, let's get into it!

1) UI
The UI would change a tiny bit less than the other games. Here's the main menu!

Not much changed as I think the FNaF 4 main menu is pretty good as it is.

And here's the new night screens, just like the other games, but now it uses teasers!

Anyway, let's get onto the gameplay!

2) Gameplay

As always, I will go over all of the characters and then discuss other general gameplay changes.

NIGHTMARE BONNIE: Activates on night 1, 12 AM. He will go to the left door. If you see him down the hallway after you flashed him, that means you stunned him, but he will get faster every time you stun and he won't get slower until the end of the night. You can stun him about 3 times before he becomes fast and then insanely fast after the 4th stun. If you hear his breath, close the door as fast as you can. He will make lights flicker if he is close to killing you. He will also bang on the door once he leaves.

NIGHTMARE CHICA: Activates on night 1, 1 AM. She will go to the right door, and the same stunning rules apply to her too, however, she will get even FASTER than Bonnie since she cannot kill you. If you hear her breath, close the door as fast as you can. She will bang on it after she leaves. If she enters, she will scream at you then break all of the lights in the house which will make everyone faster and also make you unable to use the flashlight or hear breathing, however, you can still make it to 6 AM if you're close to the end of the night. She will make the lights flicker if she is close to killing you.

NIGHTMARE FREDDY: Activates on night 1, 2 AM. Same mechanic as FNaF 4, will make the lights flicker if there are 3 Freddles. There is one more twist, if you see him standing on the bed (1st jumpscare frame!), quickly turn around and only look back on the bed after a while.

NIGHTMARE FOXY: Activates on night 2, will make the lights flicker if he is close to killing you. The closet mechanic is the same as FNaF 4, however, if you see him in one of the hallways, you will be able to stun him but only for a very short amount of time.

NIGHTMARE ENDOSKELETON: Activates on night 3. If you see his teeth while you're looking at a door, quickly flash him in order to make him go back and stun him for a very short amount of time. If you ignore him and stay somewhere else or don't use the flashlight he will grab you and you need to rapidly press spacebar to fight off against him and get out of his mouth. After this attack, the lights will flicker and he will leave. Animatronics CAN move during this sequence, as it is on you if you flash him or not. However, if he is at a door that a breath animatronic is on then he will wait and give you time until they leave so you can flash him after.

NIGHTMARE FREDBEAR: Night 5 boss, can take anybody's role. If he takes the role of Nightmare Chica he will instantly kill you. If he is on the bed, keep the light on him. He won't be just a head, but rather like how Freddy's first jumpscare frame looks like. If he is in the closet, close the door. He will flicker the lights if he is about to kill you, can be stunned but only for a very short amount of time. He can also mimic Nightmare Endo. He will speak during the entire night, and he will speak more and more as the night goes on.

NIGHTMARE: Night 6 boss, he is pretty much a harder version of Fredbear. He won't flicker the lights when he is about to kill you, his breathing is much quieter, he gives less time to search for him and react or listen and he will also speed up as the night goes on. He will speak, just like Fredbear.

PLUSHTRAP: His minigame is the same, however, if you fail it, Plushtrap will be let free into the house and he will become an enemy like the others. If you hear him run, pay attention to what direction the sound is coming from and close the right door. If he jumpscares you, you won't die, however, your vision will get red and blurry. If he jumpscares you again, you die.

As an extra feature, moving from one part of the room to another is a lot faster.
Music would play in nights 5 and 6 after 3 AM, however, I'm not sure how it would sound like.

Well, that is all for gameplay! Let's move onto lore, which has gotten the most changes in this series.

3) Lore

First of all, Dittophobia doesn't happen.
This is the Crying Child's coma. Every night is 6 minutes, but due to the pure terror that he is going through, every night feels like 6 hours.
Nightmare is the embodiment of death paying a visit to the Crying Child. His brain cannot comprehend his actual appearance, so he looks like the last thing he saw, which was Nightmare Fredbear.
Nightmare kills the Crying Child by slightly morphing into his true appearance. The Crying Child then dies, and the Fredbear Plush, which was an imaginary friend, speaks to him as if he was still alive.
The box still does exist, however, it can be opened. By collecting keys from FNaF 1, 2, 3, SL, 6 and UCN. To get the UCN key, you gotta beat 50/20. If this game was real, you'd have to get the keys as games release, so the box would only be open after UCN. The box would contain documents and contracts with dates and also MP3s of events like the bite and other stuff. Would pretty much help you solve the timeline and story at that point, and for beating such a hard mode in order to see it, I think it is rewarding enough.
The reason Crying Child even sees the Nightmare Endo is because of that time he was trapped backstage with all of those Endo heads. His minded twisted up the idea of an Endo, and then the Nightmare Endoskeleton was created.
FNaF 1 call wouldn't be in the game anymore.

Other than that, not much else changes.

4) Misc. Changes

Before I get into the Halloween DLC which was renamed to Halloween Onslaught, I will mention some other things for the OG game.
Mangle in Elizabeth's room and the toys at the playground would be removed.
Pigtail girl wouldn't tell you about the animatronics being murderous, she has no way of knowing that.
In Fredbear's Family Diner, there would be a small shop near the entrance where you would be able to see Plushtrap toys. This explains Plushtrap's origin.

Now, let's move onto...

This is the replacement for the Halloween DLC. It has been renamed to Halloween Onslaught.

This DLC adds Halloween decoration to minigames, the Afton house and other locations like menus! However, this DLC also adds a custom night.
This CN isn't like the others. Your main goal is to unlock characters by getting a higher and higher score, with the score system working like UCN. Your final goal would be to do the max mode with everyone on 20, which would give you a star.
The Characters you would get are Nightmare Golden Freddy, Jack-O-Bonnie, Jack-O-Chica, Nightmare Mangle and Plushtrap's Endoskeleton. Each one would be a harder version of the character they're based off of. As an example, Plushtrap's Endoskeleton would be a harder and faster version of the Nightmare Endoskeleton.
This max mode would also have music playing in it, due to how many characters there are. The difficulty definetly wouldn't be low.
One last thing for Halloween Onslaught, cheats and challenges! The cheats are the same as FNaF 4, but one challenge would get removed, that being the blindness one.
Also, Nightmarionne would replace Nightmare in the main campaign and Nightmare BB replaces Plushtrap in the main campaign too. This game would also carry over data from the OG game, just like normal FNaF 4.
And, no, characters wouldn't have voicelines. It wouldn't make sense, as Crying Child immediately dies once he's jumpscared.

Anyway, that is about it! Thank you guys for reading and sorry for the long wait compared to the other 3.

About the next one, I'm not sure if it should be FNaF World or Sister Location. You guys can say in the comments which one you want next!
Until then... Buh-bye!