Act 4 serves primarily as a short epilogue but also makes some interesting hints towards something even more sinister than previously suggested.
The game returns to the title screen, which is normal save the absence of Monika. As of version 1.1.0, if monika.chr is replaced before the beginning of Act 4, a dialogue box will appear before any characters speak, reading “Please stop playing with my heart. I don't want to come back.” When the dialogue box is dismissed, monika.chr is removed from the game folder if still present. Regardless, the story progresses as expected without Monika. The plot diverges from the Act 1 plot after the conclusion of the first meeting.
At this point, two things can happen, depending on the manner in which the player played the game. If the player has seen every CG in the game, then Sayori will thank the player for being so thorough, saving and loading to see every bit of content, and spending so much time with the club, and the game will progress to the credits without issue. If not, then Sayori will thank the player for “getting rid of Monika,” revealing that she knows everything that Monika did, possibly because she is the president of the club. The background distorts and flickers to the classroom from the end of Act 2 while Sayori expresses delight that she can be with the player forever, but is stopped by a dialogue box, presumably Monika speaking. Monika says that she was wrong, admitting that “there's no happiness here after all...” Regardless of if Act 4 ends favorably with Sayori or if Monika interrupts, the credits will play.