Keep the camera on Foxy's cove (cam 1C) the entire night (until Freddy laughs 5 times) and spam cams, like a lot. (I think Foxy has a movement opportunity every 7-9 seconds?) while spamming the cams listen for footsteps at the doors, close the door the footsteps came from (the audio is panned left or right depending on where it came from). after about 5 camera flips (3 cam flips on 4/20) you can check with lights and open door if they are not there. (or just open the door without checking to save power)
When foxy disappears from the cove, (which she WILL on 4/20) check the left hall, if you let her reach your door before you close it she will take 3x the power she usually would.
After you hear 5 Freddy laughs switch the camera to right door kitchen (cam 6) where Freddy should be. This helps because you don't actually have to be on Foxy's cam (cam 1C) to stall her and only have to be on cams in general.
If you're on Freddy's cam, (cam 6) she can't kill you, like at all. (Freddy also won't leave your door until you switch cams with the right door closed) and if you want to check Foxy to see if she will be running to your office soon, close right door, quickly switch to Foxy's cam, quickly switch back to Freddy, and then open right door and keep spamming cams. This way you don't waste power with right door closed for like half the night.
This is how I beat 4/20 mode first try