Ok so I want to give my own theory for what Five Nights At Freddy's 4 is going to be about based off the pictures on ScottGames.com
First of all, I believe that the game will be during the bite of 87'.
Proof: Nightmare Chica and Bonnie have the saying 'Was it me?' Nightmare Freddy says 'The final chapter.' Nightmare Foxy says 'Or me.' I believe 'Was it me' or 'Or me' is asking 'Was it me who caused the bite?' 'The final chapter' is probably meaning that it's going to be the last game.
Also, if you change your settings it obviously shows that Chica, Freddy, and Bonnie all show the word 'Nightmare' at the bottom. But Foxy has 'Out of order'