There are 5 main endings that can be achieved based on the decisions you make in chapter 4. 3 of these endings are possible in your first save slot, while the other 2 are possible in your second save slot. It doesn't matter which character you play as, as they both follow the same plot, just with different friends.
YOU DO NOT HAVE TO PLAY THROUGH THE WHOLE GAME AGAIN AFTER COMPLETING 2 SAVESLOTS!! Simply select a completed save file and replay chapter 4 up until the end, while changing your decisions in order to get the different endings.
An in depth version on how to get each specific ending:
https://steamcommunity.com/sharedfiles/filedetails/?id=24605...Once you've gotten all 5 main endings, go to the main menu in the game, click "view secrets" and then click "Main Endings." Clicking this will give you the Achievement as well as unlocking other secrets that you had not yet unlocked. You do NOT need to get all of the 14 sub endings that are possible in chapter 5, just the main endings based on the decisions you make in chapter 4.