{This is a Guide for anyone who wants to find a fairly optimal strategy, although this one still has flaws}
So I figured I'd try to help those struggling to beat the later nights, including 4/20 Mode, by giving a strategy that I used for a majority of my playtime with this game.
Quick Note:
I must mention, I beat 4/20 Mode on my third attempt by sheer luck with this method of sort: it turned 5AM the moment the power went out, and Freddy stalled long enough for me to beat the night.
Point is, good luck doing this method on 4/20 mode; with a lucky run, it is definitely possible, since I got Foxy 3 times in my run instead of the optimal 2, but that's a rare chance.
Power Guideline:
An *estimate* guideline including power taken by Foxy of the power you want to have at any given time:
1AM: 85% | 2AM: 65% | 3AM: 45-50% | 4AM: 30% | 5AM: 15%
Foxy:
Firstly, Foxy is the most important;- Always keep the camera on Booty Cove, and if Foxy comes out before 1AM, just restart unless you want to try your luck (or you are playing 4/20 Mode where it is basically guaranteed). Same goes for if you get Foxy attacking a third time, unless its 5AM in which you can use the strategy I mention below to cut her off as the power goes out.
It takes a lot of power to shut the door and trigger Foxy's dash, so I experimented and I've figured out that Foxy has a sound cue as she enters the office, which is a quick three steps (if you want to try recognize it, shut the right door and leave the left door open, waiting for Foxy to attack if you want to hear what I'm referring to- as a test).
The moment you see foxy out of her cove, do not shut the door or check the hallway. Be ready to shut the door the moment you hear the sound cue, hovering over the door button and not checking the cameras (of course, Chica could be a nuisance but it all comes down to luck, this is just the most optimal strategy for power).
Final thing I should mention: Do not refrain from checking Foxy as often as you want. If you don't check her as often as you can, you'll lose more power from her hitting the door than you will from actually checking on her.
Bonnie & Chica:
Bonnie and Chica are just random whether or not they go away, however they definitely go away quicker on 4/20 Mode.
I also should mention, I do not recommend using door lights at all unless you are checking if Chica or Bonnie have left or if you've heard the footsteps outside of the door that signal their arrival.
Freddy:
From what I've found, even on 4/20 Mode, Freddy seems to only get to the right door by around 5AM, 4AM if you are unlucky.
To make it brief, I've tested it and Freddy will attack you if you leave the door open, and sit doing nothing. If you ever check the camera, do so with the right door shut every time.
If you hear Freddy laugh again, she has went back to the previous camera to the Closet, and you are free to forget about her for, most likely, the rest of the night.
[Edit: I'm back a year later and this guide is already convoluted. I also don't want to write out another bunch of paragraphs, so for more information on Freddy, go to one of my replies under this comment. It covers some extra things on Freddy and ways to (try) and combat her.]
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[I've made this guide because this game contains elements that FNAF 1 doesn't, that can allow you to further optimize your strategy- Freddy seemingly being able to enter the office if you leave the door open too long or Foxy's sound cue as she is about to enter your office & etc.]
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After Completion of Night 6:
I don't know if this is commonly known, but following the completion of Night 6, to 100% Complete the game, you must then also click on the blue donut in the Office a few times for the "Glazed" Scene, type in '1987' on Night 7 and (I think) you must beat the night, and then finally type in '6969' on Night 7 for the Golden Freddy Scene-
(And 4/20 Mode, but that's a given)