The time has come for Pedringano's execution for the murder of Serberine. Hieronimo and a deputy enter. Hieronimo reflects on the irony of his situation, since in carrying out his official role as a judge by executing Pedringano, he will do justice for others, while still completely unable to find justice for himself or his son. Pedringano soon enters, along with some officers and Lorenzo's page. He reveals that he had written another letter asking for Lorenzo's help, but after the arrival of Lorenzo's page, realized that that letter was redundant. His attitude is now one of joviality and defiance, for he believes (wrongly) that the box contains his pardon. The Hangman asks Pedringano to step onto the gallows; Pedringano does so, confesses to his crime, but instead of showing repentance and remorse, begins to joke with the Hangman, trading witty insults, and continually dropping hints that he does not expect to be hanged; all the while, the page is continually pointing to the box, leading Pedringano on. The Hangman soon becomes annoyed, and Hieronimo is eventually forced to leave in disgust at Pedringano's impudence. The Hangman finally asks Pedringano directly whether he hopes to live; Pedringano says yes he does, by his pardon from the king. But the boy does not reveal the pardon, because it does not exist, and the Hangman immediately carries out the execution. The deputies take away Pedringano's body.
Hieronimo, having left the execution, now wanders around his estate, mourning his son. He asks where he can possibly find relief from his grief and concludes that his grief is inescapable and that it demands that he finds justice for his son's murder. The Hangman finds him and tells him that a letter was recovered from Pedringano's body, which indicates that in fact Pedringano was operating under the orders of a superior and was therefore executed injustly. Hieronimo assuages the Hangman's fear of punishment for the hasty execution and takes the letter from him. Hieronimo then reads the letter aloud. The letter is addressed to Lorenzo, and it restates Pedringano's request for help from his lord. It also contains a threat: that Pedringano will, if not aided by Lorenzo, reveal that Lorenzo ordered him to kill Serberine and also that Lorenzo and Balthazar killed Horatio with their servants' help. Hieronimo, upon reading the letter, finds confirmation for the first letter he received (from Bel-Imperia). He becomes enraged, realizing the truth of the allegations, finally convinced of the identity of his son's murderers. He resolves to seek justice from the king and heads directly toward the court.
At the Knight-Marshal's house, Isabella circles with her maid, discussing different medicines. Complaining that there is no medicine that will restore the dead to life, she has a fit, running wildly across the room. Her maid attempts to console her, but she will not be consoled; the only thing that seems to help is the thought of her son sitting happily in Heaven "backed with a troop of fiery cherubins," singing and dancing in perfect bliss. But then the thought of his murder returns to her, and she again screams out a demand for justice against the murderers of her son.
These scenes show two important developments: the increasing mental disturbance of Horatio's parents over their son's unrevenged murder, and the culmination (and simultaneous foiling) of Lorenzo's plans. These developments indicate a shift in the action, away from Lorenzo's opposing plans (which he believes, falsely, to be successful) and toward Hieronimo's plans for revenge. In other words, Hieronimo's actions now control the direction of the play.
We can see this in a change in the situations of Lorenzo and Hieronimo, regarding their respective knowledge. Lorenzo has had privileged access to the real significance of recent events. Whereas everyone else, including Hieronimo, believed that the murder of Serberine was a simple, hot-blooded killing, Lorenzo knew that it was all part of his plan. But now it is Lorenzo who is in the dark, and Hieronimo who is in the know, thanks to Pedringano's letter. And Hieronimo has now achieved certainty, thanks to yet another revelation (this time from a reliable source) as to the identity of his son's true killers.
The fact that Hieronimo was made a dupe of Lorenzo does provide some ironic distance between us and the tragic protagonist. When he condemns Pedringano to death and leaves the execution, his disgusted comments are a perfect example of dramatic irony: he consciously believes that he refers both to Serberine's and his son's murder, and he does not know that he is in fact about to hang one of his son's murderers or that in doing so he is helping his son's chief murderer escape punishment. Again, our feeling of tension and pity for Hieronimo is increased by the lack of information that propels him into this unfortunate act.