The three gulls and Mosca stand before the Venetian officers of justice; their case is about to be tried. The avocatori relate the court's shock at the whole monstrous story. Mosca introduces his advocate, Voltore, who takes the place of the enfeebled Volpone. The court is curious about Mosca's position, and Bonario describes him as Volpone's parasite, knave, and pander, concluding his tirade by demanding the fox be summoned at once. Voltore assures the court that the sight of Volpone will move their pities rather than their indignation.
Voltore begins his defense of Volpone by accusing Celia of being a "lewd woman" who has been a "close adulteress" to that "lascivious youth" Bonario. Though Corvino has often caught them in the act, his "timeless bounty" forgave them. To erase the memory of that generosity, the two lovers plotted against Corvino. Meanwhile, Bonario's doting father, disturbed by his son's heartless behavior and grieved that he could not prevent such treachery, at last decided to disinherit him.
The story is too exaggerated for the avocatori to credit; Bonario and Celia are known to be virtuous. Voltore declares they are the more dangerous for appearing virtuous. Indicting Bonario and Celia as a parricide and a paramour, Voltore accuses them of having been at Volpone's house in order to seek out and destroy Corbaccio. Celia acted as decoy to the plot, and in the scuffle, aged Volpone, bedridden these three years, was thrown naked upon the floor. The aim of the lovers was to discredit Corbaccio's free choice in Volpone and redeem themselves by laying infamy upon Corvino, to whom, with blushing, they should owe their lives.
Bonario declares that Voltore's soul is in his fee, a statement which shocks the court. Voltore patiently accepts the abuse; could a man who would not spare his parent, spare his accuser? As witness to his tale, Voltore calls Signor Corbaccio to the stand. The old crow calls his own son "an utter stranger to my loins." Bonario cannot resist the authority of his father. He is the perfect son!
Signor Corvino is next to take the stand. He declares his wife a whore and, in an effort to further his greedy cause with Volpone, describes his own cuckolding quite realistically. Mosca assures Corvino there is no shame in what he does. Poor Celia swoons at the testimony of her husband.
Mosca shows the court the wound he received in his scuffle with Bonario. Bonario realizes the cleverness of the plot for the first time.
The locale of Act IV switches to the senate chamber. The last three scenes deal with the accusation and vindication of the rogues and gulls and the prosecution of the innocent.
As the action begins, only Mosca, Volpone, and the audience are aware of the whole plot development, but even the audience is shown Mosca's plot only gradually. In this way, they can admire his improvisation and audacity. He has persuaded the three gulls to tell his lie. Mosca is undoubtedly a rogue, but he is a delightfully charming one. Like Falstaff, he must not be taken seriously, but there is much fun to be had with him.
During this sequence, it is important to visualize Mosca's movements. The gadfly darts from one bird of prey to the other, telling each that he alone shall inherit Volpone's wealth. He is not above calling each a fool to the others. Each of the three — Voltore, Corbaccio, and Corvino — plays a precarious part in Mosca's scheming.
Jonson has stylized the four avocatori, who speak in one-line sentences. They remind the reader of the silent-movie Keystone Cops, and their dress should be exaggerated.
As Voltore begins his case, it is evident that he has an excellent insight into the human nature of the court. His patience with innocent Bonario is an artful reversal of character values. In view of it, Voltore's own foolishness is delightfully underlined. He is eloquent in defense of his client. The irony lies in the fact that as his professional skills are most subtle and effective, his personal foolishness is most evident. This is equally true of the other two gulls. Corbaccio is convinced his words are preserving his son's inheritance when they are actually ruining the young man. Corvino saves his reputation by declaring his wife a whore and himself a cuckold! Mosca's plot is an excellent use of the materials chance handed him. Finally, Mosca's dexterity in moving calmly from one gull to the other without mixing up his story is worthy of applause. On the stage, the smoothness of these transitions contributes to the pace of the hilarity. Jonson understood what performers might do for this action.
Bonario remains in character throughout the proceedings. He is an ass, and he is unaware of the plot until it is too late. Always in character, Celia manages a very feminine swoon. Neither of the innocents helps the cause of justice.
The reader should be aware of this scene's excellence as courtroom drama. The members of the audience have not heard Mosca's composition, but they know his desperate position. Will justice triumph? Will the avocatori see through Mosca's lie? How will Bonario and Celia escape from their predicament? Perhaps the audience senses that Mosca and Volpone will be unmasked. Certainly the comic tradition of the Elizabethan stage demanded their downfall. Still, the important questions of what will undo them and how it will be accomplished remain to be solved.