Managed to beat 4/20 and 100%, it somehow was easier than night 6.
If anyone is still struggling, I'll try to give some informations that helped me a lot once I understood/found them. It's a huge wall ass of text, but it's imo better than 1 single tip that doesn't work alone.
Quick summary : Wait between 5~6secs then look at her cam for 3secs, don't look at any other cam and not when it's disabled by Bonnie entering/exiting that cam, keep both door in sigh on you won't hear both Bonnie/chica, when any of these two attacks close the door then look at Foxy 3 seconds then open without wasting energy on light, wait for Foxy footsteps to close door when she attacks, count 30seconds when reaching 5a.m then close the right door. More info on each girls below and to 100% if you want to understand more.
For Foxy : count 5 ~ 6 seconds in your head, check her 3 seconds (saw it another comment, worked for me), repeat until you have <10% power. Don't check too often from here, but still 3~4 times (for 1 second no need for more to make sure she's not attacking), unless she's already on all four outside the curtains, repeat the 5/6 secs + 3secs until 2~3% power. When you hear Bonnie/chica footsteps, try to wait until there's the third footstep, close the door, check Foxy 3 seconds, and re open the door (no need for light). VERY rng, haven't been f by Chica nor Bonnie by doing this timing, but Foxy still advance stages, mostly when interacting with doors/lights. If she attacks before 1 a.m, reset the night. You want her to attack twice during the whole night, a third time only if it's to break your last 2-3% (she won't kill even if the first hit make you loose all your energy.) When she attacks, DONT look at the other cam at the left of your room, it'll make her arrive instantly. Just wait, cursor on the button, and when you hear her footstep close, she 3 hit it, open (unless bonnie attack in the same time, which happens a lot, use the light to make sure she leaves with the noise).
For Bonnie (and Chica, they're very similar): Don't bother looking at them on cam. Put the volume high, keep your vision in the middle of the room (to have both door visible, if you're full right/left you won't hear any steps coming from the other side). For 4/20, they leave (and attack) in very short time (making it easier than night 6 since you don't have to wait 3 years for Bonnie to leave, draining your power faster than your cum.) In case you want to make sure Bonnie is here, DONT use the light to verify their presence when hearing footstep, close the door, and when you switch the light you'll hear a noise if she's behind the door for the first time per attack. Very RNG as their attack are quite random, the more their attack the less energy you'll have for Freddy, I somehow got a near perfect first hour where none of these two attacked. Important : when Bonnie enter/exit Foxy cam, she'll disable the camera : from what I've seen, it's the same as if you're not looking at Foxy, so don't even bother wasting time even if you haven't done the whole 3 seconds, just save power, wait a few seconds, and count 3 seconds when the camera is enabled.
For Freddy: She's the most biggest threat as she's the one that ended all my tries. She laughs when she moves, and when next to your room, her next laugh means she's attacking, and her next one brings her back 2 room before. If your right door is open at this time, you'll get f. Once you reach 5 a.m, count 30 seconds (yes, 30, after the 0.0.7 she's slower at attacking), close your right door. If you have 0% power before that, she'll get you. If you hear her laught after closing the door, keep it closed and you can consider it a victory (see why next), and if you reach 0% power but hear her laught in the next few seconds, it's also a victory. When power reach 0, Bonnie, Chica and Foxy can't do anything to you, even if you hear their footsteps. Freddy will f you after a set amount of time if her attack doesn't start after hearing the lullaby. The lullaby cuts very quick if her attack starts after 5~10seconds, she'll f you. But! If you managed to hear her laught before the 0% or a few seconds after, AND if you don't move your cursor (didn't try moving it but if you reach that point don't bother loosing just for science please), the lullaby last longer and you'll win during it/before Freddy f you. It even happened to me that I reached 6a.m when she appeared on my screen, saying that sexy rawr before the 5.am to 6.am screen appeared.
Spoiler on 100% here:
You have as expected 2 scenes with each girls, needing to game over from each one to unlock each one in gallery, second scene by beating night 5 (giving first start)
The blue donut looking thing in game when you're not looking at camera unlock you one scene and some % when clicking multiple time on it
Clicking on Freddy boobs on the post multiple time give you some % and a little spoopy forced closed game
Beating night 6 for second star and unlocking night 7, allowing you to enter "passwords" and doing the 4/20 (all 4 girls at 20 levels)
Beating 4/20 for third start and the orgy scene while unlocking the 4 plushes (that foxy one is so cute I wish she had balls that huge compared to her body >//>)
Night 7 with each girls level set as 1 - 9 - 8 - 7 in that order AND BEATING IT unlocks you 2 cute plush (that don't do anything exept being here and granting some %)
Night 7 with each girls levet set as 6 - 9 - 6 - 9 gives you the same spoopy jumpscare as clicking on freddy boobs, except here you have also a sex scene (not accessible in the extra after that, but since you just need to lauch a night to reach it it's the same) ; trying to get back to menu multiple time close the game. It also gives you the final 7th plush.
What don't give some % but are here (mostly easter eggs) :
some characters appearing on some cams ; some poster changing the picture/text ; there is something related to markiplier but haven't found it myself ; 1 / 100 000 each sec that looking at the cam on the left of your office have the poster changed to golden freddy (if following fnaf 1), giving you a game over if you close cam while it's her, to prevent that just switch cam and it's all good (if trying to beat 4/20 you'll never need to look here don't worry, just stay on foxy)
and most certainly other things I've not found
I'm not sure if that counted toward 100%, but I've also beaten 8 - 0 - 0 - 8 ; all disabled night. No need to do a night with a single one enabled at 20.
This is a very, VERY long comment, but I feel like it'll be useful to people still tring to beat 4/20 (and any night honestly, even if bonnie sucking the door for 30 seconds doesn't help with the power at all at lower nights), and to understand a bit more how the game works. It took me 6 tries to beat 4/20 using that strategy.