My theory is that fnaf 4 is before the events of fnaf 2.
Where will it happen: A freedy fazbear pizzeria , the original one.
When will this game have its events?: Before fnaf 2, right after the murder.
Who will I play as?: Most likely a new security guard or the Purple Man.
Why the Purple Man? Its simple, the purple man was a security guard and had the power to open all the DOORS and manipulate animatronics. ( As seen in the SAVE THEM minigame) as for he being able to open doors, most likely him being a employee for a long while. This let him take the suits and use the Freddy , Chica, Bonnie, and Foxy animatronic ,to Kill the children (Not springtrap because Springtrap kills instantly, but we never saw a Wearer of the Golden Fred suit die inside)
What about the Nightmare Animatronics? To ensure the harmless animatronics would kill (lets face it, without the dead spirits they were harmless because their A.I wouldnt kill) he turned spare suits into the Nightmare animatronics, leading to the huge killing. Although Purple Man wanted some for himself, and killed the kids in Pirate Cove as well.
What about the "Borrowed yellow suit"? That was when the storgae room was found with the Purple Man's Body inside of Springtrap, so they boarded it up.
Any loopholes you guys find I'll just edit the theory to work with it (Im sleepy give me a break ;-;)
Thanks for reading!