The truth is 4/20 is not much more difficult than off/off/off/20 xD Because Foxy is so buff that it takes 90% of difficulty anyways. I was finishing Foxy-only runs with 1-5% power left tops (but typically just last foxy raid drained my power to 0 and survived only because ending sequence took so long that I was getting 6AM before Freddy finished her shenanigans) and tbh got exactly the same result with 4/20. You just gotta ignore Chicka and Bonnie and block Freddy precisely on time using stopwatch during gameplay (she appears precisely at 7:42 so close the door 7:38 and reopen after hearing laugh) and that’s it. I was ignoring Bonnie and Chicka probably more than would seem reasonable - I didn’t even check if they’re gone just YOLO reopened doors after 3 seconds. Used light when opening only occasionaly jus to check if maybe I shouldn’t but near the end of run when I was low on power I didn’t do even that just hoped they’re gone.
Just don’t assume you’re gonna survive with power left because you won’t. Realistically your task is to survive up to around 7:50 in the run (which is 8:35 long) because typically freddy ending sequence should take 30-40 sec and that should get you to the 6AM. I got 6AM literally split of second after Freddy already said “rawr”.