Act IV scene 1 begins in front of the altar in the church. However, instead of the wedding that everyone expects, Claudio denounces Hero as a "loose woman." Leonato is furious, supposing that Claudio was the one who has slept with his daughter, but Don Pedro and Don John second Claudio's charge. A furious and distraught Claudio questions Hero about what he saw and the man she was supposedly with. Hero rejects the accusation. Upset, Claudio says he will never love again. Hero is overwhelmed by the events on her "wedding day" and swoons as Claudio, Don John and Don Pedro leave the church. Leonato believes the accusation and say that if Hero is not dead he will, with his own hand kill her. However, Friar Francis supports Hero and Benedick guesses that Don John might have misled his brother and Claudio. The Friar therefore tells Leonato to hide his daughter away and to publish news of her death to try and make Claudio feel compassion towards her.
Benedick and Beatrice find themselves alone and admit their mutual love. Benedick foolishly asks for Beatrice to ask something of him to prove his love for her. Beatrice replies asking for him to kill Claudio. Reluctantly, he says he will fight a duel with Claudio.
Act IV scene 2 occurs in the jail, where the incompetent Dogberry interrogates Conrade and Borachio. They deny all charges, but Seacoal say they heard Borachio saying he received payment for falsely slandering Hero. The sexton then shares the news that Don John disappeared after the accusation of Hero and Hero has died with grief. The accused men are to be bound and brought before Leonato. Conrade insults Dogberry, saying he is an "ass." Dogberry is shocked, but insists on everyone recalling that he is an ass, although a written testament of this will not be created.