Volpone returns home after the drama at the Scrutineo, tired. He declares that he has grown tired of his con and wishes it were over. Pretending to be sick in public has made some of the symptoms he has been falsely presenting, such as cramps and palsy (tremors), feel all too real. The thought that he might actually be getting sick depressed and frightens him; to banish it he takes two strong drinks and calls Mosca.
Volpone calls Mosca and informs him that he wants to be over with the con. They discuss how well the entire con went off and congratulate themselves on being so erudite, so brave, and so clever. Mosca advises that Volpone should stop his life of trickery here, for he will never outdo himself. Volpone seems to agree, and the begin discussing the matter of payment to Voltore for his services, something that Mosca insists on. But Volpone suddenly decides to carry out one final joke on the legacy hunters. He calls in Castrone and Nano, and tells them to run through the streets, informing everyone that Volpone is dead. He then tells Mosca to wear his clothes and to pretend that Volpone has named him the heir to the estate when the legacy hunters arrive, using an authentic will naming Mosca as heir. Mosca remarks on how distraught all four of the people involved in the deceit at the Scrutineo—Voltore, Corbaccio, Corvino, and Lady Politic—will be when they come to believe that Mosca has been chosen over them. Soon, Voltore arrives, and Volpone hides behind a curtain.
Voltore enters to find Mosca making an inventory. Thinking that the property is now his, he praises Mosca's hard work. He takes the will in order to read it. Corbaccio, clearly near death, is carried in by his servants. Corvino soon after enters, and soon Lady Politic Would-be enters too. All the while, Mosca continues to take an inventory of Volpone's property. All four characters then read the will; they understandably react with shock, and demand an explanation. Mosca replies to each of them in turn, reminding them in a short speech of the lies and other immoral acts each of them committed. Lady Politic apparently offered to provide Mosca with sexual favours in return for Volpone's estate. Corvino, of course, unjustly declared his wife an adulterer and himself a cuckold; Corbaccio disinherited his son. For Voltore, Mosca is somewhat sympathetic; he expresses sincere regret that Voltore will not be made heir. After Mosca is finished to talking to a character, that character leaves. After Voltore leaves, Mosca and Volpone are again alone, and Volpone congratulates Mosca on a job well done. Volpone wants to gloat directly in the faces of the four dupes, so Mosca suggests that he disguise himself as a commandadore (a sergeant or guard), and approach them on the street. Volpone congratulates Mosca on his excellent idea.
The intention of Jonson throughout the play has been to satirize greed in all its forms. At first, Volpone was the instrument of Jonson's satire; he turned the greed of the legacy hunters against itself, creating a situation where greed resulted in not only a complete loss of dignity on the part of the legacy hunters but also, ironically, the loss of the very thing they were seeking to gain: money. But now, Volpone has succumbed to his own form of greed; greed driven by his private desires and appetites for Celia. Because of this, he has defamed two innocent characters, Celia and Bonario. In the moral universe of Jonson's comedy, this transgression cannot go unpunished or uncommented upon; Celia and Bonario were guilty of nothing except dullness; their imprisonment is, to put it simply, "not funny". So Volpone is no longer the instrument of Jonson's satire. In fact, he is now made the target of it, and the attack proceeds, again, through irony.
A central motif in the final act is that of the disguise-made-reality; Volpone has convinced so many people of his lies that his falsehoods now come to stand in the public sphere as truth, with terrible consequences for Volpone. Volpone wishes to be done with his con-game clearly indicates his wish to be done with his con-game, but we receive indications that it will not be so simple, that the lies Volpone has told are too powerful and too widely accepted to simply disappear. He returns from the senate complaining of cramps and aches that roughly coincide with those he has been imitating; the "cramp" and the "palsy," which he had mocked Corbaccio for succumbing to in Act I. These may be indications of a guilty conscience; but they also stand as a metaphor for the way in which Volpone has successfully blurred the line between lies and reality. Again, we can use the metaphor of stagecraft here: in Act IV, Volpone crosses boundary between the "stage" (Volpone's private life) and "reality" (the public realm of the Scrutineo), by carrying his "play" into the world and appearing sick in public. Ironically, it is at this moment that Volpone impulsively decides to kill himself off, and he does it using the medium of the playwright, the written word (the will).
So when Volpone thinks he is writing himself out of his deceitful game, his "play," he is actually writing himself out of reality altogether. The "exit from reality" occurs when Volpone goes behind the arras, he for a moment becomes a member of the audience of Volpone, the drama written by Ben Jonson; in other words, he is a spectator, not a participant, in his own life. Mosca, at this stage, assumes Volpone's role both as the center of the play's action and as its (admittedly dubious) moral voice; it is he who scolds each legacy hunter in turn for their hypocrisy. Volpone delights-almost sadistically—in the vindictiveness with which Mosca reminds each character of the callous and immoral acts they committed in the pursuit of Volpone's treasure. But the irony of the situation is encapsulated by Volpone's statement "Rare, Mosca! How his villainy becomes him!" which foreshadows the events later in the act.