After several hours of practice, I was able to beat 4/20 Mode! My strategy was to look at foxy for as long as possible, using audio to keep track of Freddy's movements (laughing), and close the doors in the other's faces when I heard footsteps. I learned that if you check the hallway as Foxy runs towards you, you preserve much more battery as they bang on the door.
I now have 100% completion, seeing Golden Freddy, dying to all 4 characters (2 scenes each after night 5), and the Doughnut scene, and beating all modes. After beating 4/20, and getting GF and Doughnut scene, be sure to BEAT 1987 mode on night 7 to get 100%, and unlock a nice little reward~