The scene is now set at the Scrutineo, the law courts of the Venetian state. Voltore, Corbaccio, Corvino, and Mosca enter. They are about to appear before the Scrutineo to answer the accusations of Bonario and Celia. Voltore is the one who will present the case, since he is a lawyer; Bonario expresses concern to Mosca that Voltore will now become co-heir because of this service to Volpone, but Mosca assures him there is nothing to worry about. He also worries that his reputation will be ruined in front of the Scrutineo (presumably because of his decision to prostitute his wife). Mosca assures him that he has given Voltore a story to tell about the incident that will save Corvino's reputation. Mosca also lets Voltore know that he has another witness to appear if necessary, but he doesn't say who it is.
The four Avocatori (who serve as judges in the Venetian state) enter, along with Bonario, Celia, a Notario (Notary) and some Commandadori (guards). The Avocatori discuss how they have never heard anything as "monstrous" as the story Celia and Bonario have just told them: that Corvino agreed to prostitute his wife to Volpone in the hopes that Volpone would make him heir, that Volpone tried to rape Celia and that Corbaccio disinherited his son Bonario. They demand to know where Volpone is: Mosca replies that he is too ill to come, but the Avocatori insist that he come anyway and send some of the Commandadori to fetch him. Voltore then begins to speak to the Scrutineo. He tells a very different story from that told by Celia and Bonario. He claims that Celia and Bonario are lovers; that Bonario went to Volpone's house with the intent to murder Corbaccio for disinheriting him, but finding him absent, decided to attack Volpone instead; and that Celia's cries of rape were part of an attempt to frame Volpone devised by her and Bonario, in order to prevent Volpone from collecting his inheritance. Voltore then produces the "proofs" of his story. These consist in the testimony of Corbaccio and Corvino, who corroborate the story, with Corvino adding that he has seen Bonario and Celia making love with his own eyes, and that he has their love-letters in his possession (which in reality are forged). Mosca further adds that he was wounded while defending his master. Celia faints; Corvino accuses her of acting. The Avocatori begin to express doubts about Celia and Bonario's story. Then Mosca informs the court of his "surprise witness"; she is a "lady", who saw Celia in a gondola with her "knight". He leaves to fetch her, as the Avocatori express their shock at the turn of events.
Mosca enters with his surprise witness, who is, of course, Lady Politic Would-be. She corroborates Mosca's claim, hurling abuse at Celia. She then apologizes profusely to the judges for disgracing the court; the judges attempt to assure her she has not, but can't get a word in edgewise. Voltore then produces his final "proof". Volpone enters, looking old and crippled; Voltore ironically comments that they can now see Celia and Bonario's rapist and criminal. Bonario suggests that Volpone is faking (which he is), and should be "tested", which Voltore takes to mean "tortured"; Voltore ironically suggests that torture might cure Volpone's illness. The Avocatori are convinced of Voltore's story, and demand that Bonario and Celia be taken away and separated. They apologize to Volpone for disturbing him, and express outrage at the "deceit" of Bonario and Celia. Mosca then congratulates Voltore on his work. He assuages Corvino, who is still worried that Voltore will get part of Volpone's fortune. And he demands that Corbaccio pay Voltore. Corbaccio and Voltore leave, and Mosca then assures Lady Politic Would-be that, due to her support today, she will in fact be made Volpone's principal heir.
The Fourth Act is marked by Volpone's near complete disappearance for the play; Mosca takes his place as the driving force behind the plot. Though Mosca has been central throughout the entire play, in the Fourth Act he truly becomes an independent character, arranging to have Lady Politic Would-be testify against Celia. Volpone's absence in the Act can be seen as a symbol of the growing distance between him and the audience; with his attempted rape, he gives up his claim to our sympathy, and this is symbolized by temporarily giving up his place in the play. Mosca fills the vacuum left by Volpone's absence; and his sidekick role is in turn taken up by Voltore. This shift in the focus of the play emphasizes Mosca's independence from Volpone; Mosca now can carry the plot by himself. And this increased independence from Volpone, in terms of the ability to drive the play forward, foreshadows the play's next Act, where Mosca will actually try to usurp Volpone's role in society. Mosca and Voltore's triumph over Celia and Bonario in The Scrutineo represents the triumph of stagecraft over truth. We can think of the Scrutineo as the stage on which they operate. The Scrutineo was the Venetian Senate-building; the Senate was the head governing body of the Venetian state. As already discussed, the Venetian state was a symbol of decadence and deceit; and the Scrutineo, as its center of power, would have had a very strong association with illusion and deceit. Furthermore, it is easy to imagine the scene being staged in such a way that the audience in the theatre (watching Volpone) could become part of the audience at the Scrutineo, thus making the audience direct spectators of the drama unfolding between the characters, and turning the Scrutineo into an actual theatre, with real patrons.
The way Voltore and Mosca go about creating their illusion has similarities with the way playwrights go about creating theirs, using words and images in a dramatic manner. They do not simply tell a lie; they tell a story. Voltore weaves a tale for the Senate full of characters one might expect to find in a sensationalistic play; the treacherous wife (Celia), the murderous, deceitful, son (Bonario), the innocent, betrayed husband (Corvino) and the deceived father (Corbaccio). Corvino's frequent interjections of salacious details about Celia- "these eyes/Have seen her glued unto that piece of cedar / That fine well- timbered gallant" increases the dramatic tension of the scene, which culminates in a couple of suprising "plot twists": Lady Politic Would-be's condemnation of Celia and Volpone's sudden arrival, looking ill and impotent. The objections of Bonario and Celia are incorporated into Voltore's narrative, much like the villain into the plot of a play; Voltore uses verbal irony, a device Jonson loved, to ridicule Bonario's suggestion that Volpone be tested for deceit: "Best try him, then, with goads or burning irons; /Put him to the strappado: I have heard, / The rack hath cured the gout." Bonario's comment is framed as just the type of thing a murderous, sick individual like him would say—just the type of dialogue that he would speak. The audience of this play within the play is composed of the four Avocatori, and their increasing anger mirrors our increasing anger; except that we know their anger is based on false beliefs. When one judge observes that "'tis a pity two such prodigies should live," his statement is an example of dramatic irony. He intends to refer to Celia and Bonario, but we know that the statement much better describes Volpone and Mosca. A careful reader would note, however, that in feeling angry at Volpone and Mosca, we are being drawn into a certain reality in much the same way that the four judges are—through images and words, arranged in a dramatic manner, good characters vying against evil ones, drawing our sympathy, making us involved in their struggle. It could make us very suspicious of the exercise of drama as a whole. Drama seems based on the very same methods of deceit used by Voltore and Mosca. But to say that Voltore and Mosca are dramatists is not to say that all dramatists are like Voltore and Mosca. Jonson, after all, acknowledges in his dedication that many dramatic poets rely on sensationalism to sell their plays, plays that harm the moral good of society. This scene can be viewed as an exercise in how to spot this sensationalism, how to differentiate between the good play and the bad play; plays that deceive and confuse; and plays, like Jonson's, that aim to tell the truth.