Sir Politic and Peregrine are walking along a canal, and Politic undertakes to teach Peregrine a thing or two about life in Venice. His two main points are that one should never tell the truth to strangers, and that one should always have proper table manners, which Politic then goes on to explain in full. Contradicting his first bit of advice, Politic then tells Peregrine about several moneymaking schemes he has in the works. To begin with, he plans to supply the State of Venice with red herrings, bought at a discount rate from a cheese vendor in another Italian state. He also has a plan to convince the Council of Venice to outlaw all timber-boxes small enough to fit into a pocket (in case a disaffected person might hide gunpowder in his or her tinderbox), and then supply the larger tinderboxes himself. His last great idea is a "plague- test," to be administered on ships arriving from the Middle East and other plague-infected areas so that they might not have to undergo the usual fifty or sixty days of quarantine. The plan involves blowing air through a ship from one side, while at the same time exposing the crew to thirty livres worth of onions cut in half from the other side; if the onion changes, color, then the crew has the plague. Politic then make an off-handed comment about how he could, if he wanted to, sell the entire state of Venice to the Turk. Just so Peregrine will know everything about his personal life, Politic lets him read his diary, which includes every single detail of Politic's day, including his decision to urinate at St. Mark's cathedral.
Lady Politic, Nano, and some serving women enter, looking for her husband. Sir Politic's wife complains that his unfaithfulness is ruining her complexion. They suddenly see Politic and Peregrine together. They meet, and Sir Politic introduces Peregrine to Lady Politic. But Lady Politic assumes that Peregrine must be the prostitute of whom Mosca was speaking, disguised as a man. She rails against her husband for his unfaithfulness, while he reacts with complete and utter incomprehension. Peregrine asks Lady Politic to forgive him for offending her, though he has no idea how he has. When he begins complimenting Lady Politic's beauty, she reacts with suppressed outrage.
Mosca enters and finds Lady Politic incensed over her husband's infidelity. She explains to him that she has found the prostitute he mentioned in Act III, and points out Peregrine. Mosca then explains that she is mistaken. The real prostitute (according to him) is currently at the Scrutineo (he is referring to Celia). Lady Politic then apologizes in a very sexually suggestive way. Peregrine is now incensed, for he thinks that Sir Politic is trying to prostitute him to Lady Politic, and vows that he will get revenge.
The moral satire of the play becomes somewhat submerged in the Fourth Act, as considerations of plot and tone become more important. Jonson frames the intense confrontation between Volpone, Celia, and Bonario with humorous scenes involving the Politic Would-bes. These scenes help keep the tone of the play somewhat light. We have further development of Sir Politic's character in IV.i; he is not only now vain, he is also greedy. But he is greedy in a completely non- threatening way, and his plans are laughably far-fetched. In a way, he is a very sympathetic character, and he is always the one who pays the greatest price (in ridicule) for his vanity, such as when he has Peregrine read his journal for the day, one of the funniest sections of the play. Only someone with a great degree of self-obsession would record such gems as "I threw three beans over the threshold" and then "at St. Mark's, I urined," and to then expect others to find it interesting. But he doesn't seem to mind, because he doesn't seem to know, that people find his behavior ridiculous.
For her part, Lady Politic outdoes herself by taking Peregrine to be the courtesan described to her by Mosca and then coming on to Peregrine as soon as Mosca informs her that he is a man. Again, the tone of the play veers toward farce. Several plot considerations are satisfied in this part of the play as well. Jonson needed to bring the subplot towards a resolution, which he does by giving Peregrine a reason to be angry at Sir Politic: the lustful behavior of Sir Politic's wife. It seems something of a weak reason, especially by today's standards. We also see, in this section of the play, the increased importance of Mosca; Mosca is the one who sets up Lady Politic Would-be to identify Celia (the woman he refers to as being before the Senate). In fact, Mosca will come to dominate the act, along with Voltore.