When thinking of new enemies, I realized that it should try to solve the problem of: in high level play you only check 1 camera: the one right in front of your room. Not only do you not have enough time to check out the other cameras, everyone passes by the room in front of you, which means that's the only one need you to check. This means this 5th enemy (that I only just thought of) should completely ignore the center room and can only be found on other cameras.
I've managed to beat the game. I actually ended up making the enemies more aggressive (if all of them are set at the maximum difficulty) because I felt I could beat it. The hardest part of the challenge is actually to stay concentrated for 3 minutes in a row, cause one mistake will end it. But with the extra time I have, I guess I can consider adding more enemies. I'm not sure I will (the sound and light mechanics are built around having those 4 enemies in particular), as I don't want an enemy that feels forced, but maybe there's something else I can do.
What's funny is that other than tweaking the aggressiveness of the enemies, this game is basically done. I guess Five Night's at Freddy's games really aren't that difficult. Which means instead I can try to add some polish, like finding actual sound effects (it feels weird when you activate sound in the game and you hear nothing) and doing some lighting.