Board Thread:General Discussion/@comment-3542887-20170717225144/@comment-29644557-20170718191128

Well, FANF 3 is Phantom Animatronics, which are hallucinations caused by oxygen deprivation. They can't kill, they only make your systems go haywire.

FNAF 4 is Nightmare Animatronics, which are literally monsters in the protagonist's nightmares, and their "killing" is representantive of the protagonist dying in real life. So those are the ones that kill. The ones from 3 and 4 are different.