Re: Chapter 4 alternate ending:
Make lots of mistakes when you're playing as Satoshi trying to save Naomi, allowing her to die. Then, before talking to Tohko in the infirmary, make sure you find Seiko's Cell Phone in the 1F hallway. Tohko will try to stab you, but her scissors will hit the cell phone in your front pocket, saving your life. The alternate end will follow from there -- though I find that particular alternate end to be the most confusing in the game, so good luck making heads or tails of what happened or why!