Act 4 opens in Rome, where Monticelso asks Francisco what he plans to do about Isabella's untimely death by poison. Francisco explains that he wants to avoid war with Brachiano, since that would cause suffering to his land and people. Instead, he intends to leave revenge to a higher power. Monticelso admits that he has a book which contains the names of all the criminals in the city, and offers to show it to Francisco so they can find an ally. When Monticelso leaves to retrieve it, Francisco admits to the audience that he will seek revenge, but that it will be slow and patient.
Monticelso returns with his "black book," and lists the types of criminals detailed in it, including: informants, pimps, pirates, female cross-dressing pimps, moneylenders, and crooked lawyers. He then leaves Francisco with the book, who, in an aside, accuses Monticelso of using the book for corrupt purposes. Nonetheless, he resolves to find and hire a murderer, and imagines Isabella's face to keep himself motivated.
Isabella's ghost appears to Francisco, which he interprets as a result of his active imagination and his melancholy. Francisco briefly interrogates the ghost, but ultimately decides such a pursuit is folly. Isabella's ghost leaves, and Francisco returns to planning.
He arrives at a plan, which involves pretending to be in love with Vittoria. He writes a letter to her, and then orders a servant to deliver it to either Vittoria or the matron of the house of convertites, provided the hand-off is done before Brachiano's men. Francisco then remarks to himself that he will only trust that Brachiano is dead when he is able to play a game with his head. Since he cannot appeal to the gods in heaven to achieve this purpose, he will now appeal to the gods of hell.
At the house of convertities, Flamineo bargains with the Matron in an attempt to secure visiting rights for Brachiano. When Francisco's servant arrives with the letter, Flamineo offers to deliver it to his sister, and the servant acquiesces.
Brachiano enters, and insists upon reading the letter to his beloved. Flamineo reads the love letter aloud, along with insulting commentary. Brachiano believes Francisco's ruse, and thinks that Vittoria has been untrue. He angrily searches for that "whore," seeking to destroy her. Flamineo is taken aback at the insult to his sister, and when Brachiano threatens him, Flamineo reminds him of his culpability in Camillo's murder. The two insult each other, and Brachiano again demeans Flamineo as a low-class pimp who is unwise to challenge his superior.
Vittoria then enters, and Brachiano confronts her about the letter. Vittoria denies having any other lovers, and she claims the letter is a deceit intended to separate them. Brachiano doesn't believe her, and in an action mimicking his divorce from Isabella, he grabs Vittoria's hand and proclaims them separate. Angry and insulted, Vittoria insists she will now live to make the whole world regret smearing her name. She promises Brachiano that God will punish him for Isabella's murder, and then falls weeping on her bed.
Her attitude shocks Brachiano, who remembers his love for her. He tries to comfort and woo her, but she resists, and curses Flamineo for having helped him. Flamineo argues for her to forgive Brachiano, comparing her to a young hare that cannot run for very long (or cry, in Vittoria's case), and so must eventually crouch in submission. He then advises Brachiano to fondle and kiss her, which he does.
Vittoria gives in, and Brachiano pledges to never again accuse her of being a whore. He further resolves to smuggle her from the house of convertites to Padua, which he insists will be an easy escape since all of Rome is in turmoil due to the Pope's recent death. He promises to bring Flamineo, Marcello, and Cornelia with them, and to make Vittoria a duchess in Padua.
Flamineo interrupts him, however, to tell a fable of a crocodile who had a toothache caused by a worm stuck in its teeth. A little bird helped the crocodile by removing the worm, but the crocodile, afraid the bird might advertise the fact that the crocodile didn't pay for the service, tried to eat the bird. The bird, however, pecked at the crocodile's mouth until it opened its jaws and the bird could fly away.
Brachiano interprets the fable to mean that Flamineo wants to be rewarded for his services, but Flamineo explains that the crocodile represents Vittoria and the worm represents her tarnished reputation. Brachiano is represented by the bird, for he has helped her improve her reputation. Flamineo cautions Vittoria to avoid ingratitude to Brachiano, and to forgive him for his insults. Aside, Flamineo justifies his changeable role of knave, madman, and wiseman, by explaining "knaves do grow great by being great men's apes," i.e. by pretending to be great men.
Lodovico, Gasparo, and six Ambassadors enter. Francisco enters through a different door, and asks Lodovico to guard the room where the cardinals are discussing whom to elect as Pope. The ambassadors, "knights of several orders," pass across the stage wearing beautiful costumes after having pleaded with the cardinals on behalf of their various countries. Lodovico checks all of the meals going into the cardinal's conclave for bribery or solicitation letters, explaining to the ambassadors that, until they elect a Pope, no one is allowed to petition them. Francisco proposes a bet with Lodovico over how long the cardinals will take, but before they can finalize the bet, the Cardinal of Arragon appears on the terrace with the announcement. Arragon speaks in Latin, explaining that Monticelso has been elected the next Pope, and he has chosen Paul IV as his name.
At that moment, a servant enters to alert Francisco that Vittoria has fled the city with Brachiano and Giovanni. Francisco orders the servant to apprehend the Matron, but then explains privately to Lodovico and the audience that he has planned precisely for such an escape, since it will give him opportunity to enact his revenge on Brachiano away from Rome.
Monticelso enters in his papal robes and, after hearing the news from Francisco, excommunicates Vittoria and Brachiano. Everyone on stage exits except for Francisco and Lodovico, who discuss their plan to murder Brachiano.
Francisco leaves as Monticelso re-enters. The new Pope asks Lodovico why Francisco worked so hard to arrange his pardon, and accuses the former of working for Francisco in some illicit capacity. Lodovico claims he has been brought to help break a stubborn horse, but Monticelso refuses to believe it. Lodovico then reveals, in a binding confession, that he loved (or at least lusted after) Isabella, and has promised Francisco that he will avenge her death by murdering Brachiano. Monticelso curses him, warning him that he will fall into ruin if he persists in such evil.
Monticelso exits, and Lodovico admits his confusion over the Pope's reaction, since he has such reason to hate Brachiano. Francisco enters with a thousand ducats from Monticelso as a gift, and Lodovico realizes that Monticelso has to hide his motives because of his new position.
Monticelso provides one of the most damning examples of Webster's theme of the difference between appearance and intention. This is clear even before Monticelso is named Pope. He brings Francisco his "black book," well aware that he is facilitating murder, even as he continually advises Francisco against revenge. His disgust for murder is doubly harsh once he is named Pope, even though he continues to fund and aid the purpose. Lodovico describes this contrast through the metaphor of a blushing bride. Like a "virgin" bride, Monticelso must appear innocent and chaste, but both he and the bride harbor improper desires behind their facades. Because he is affiliated with the Church, his hypocrisy serves as Webster's indictment not just of humanity but of the institution of the Catholic Church.
Though Francisco is committed to murder, his motives are more complicated. The scene with Isabella's ghost reveal his split sentiments. He initially asks the ghost how it was killed, but then dismisses it as a result of too much bile in his system. He alludes to the concept of the four humors, which is the pseudo-scientific idea that moods are ruled by a balance of four bodily fluids, or humors: black bile, yellow bile, blood, and phlegm. If someone had an imbalance of humors, their personalities would shift towards that of the humors (melancholic, choleric, sanguine, or phlegmatic). When Francisco successfully rejects Isabella's ghost as merely a physical symptom, The White Devil departs from the traditional revenge tragedy structure. In other revenge tragedies, like Shakespeare's Hamlet, ghosts are taken very seriously, and usually utter illuminating truths. Francisco doesn't care about what Isabella's ghost has to say, and the reader is forced to question Francisco's motives for the revenge. Does he really care about his sister's death, or is he doing this for other reasons? Is he trying to please a supernatural sense of justice, or is he merely inspired by his own earthly feelings?
Misogyny continues to perpetuate in this Act, most of all through Brachiano and Flamineo's behavior towards Vittoria. Though she has suffered much for their relationship, Brachiano is quick to accuse her of betraying him. So deep is the fear of cuckoldry that he immediately lashes out from anger and fear. On top of his exit from her trial, we see that even this deep love is less important that his social superiority as a man. In terms of traditional revenge tragedy, Brachiano would function as the tragic hero, but by this point in the play, it is difficult if not impossible to allow him such a noble title.
Flamineo sees his sister less as woman than as tool towards his betterment, but his fable reveals that he will use the attitudes towards women to his advantage. Knowing she is capable of causing great harm while still essentially powerless, he does everything he can to ameliorate the situation. He does not want to empower or punish her, but merely to keep her in her place so that less trouble is caused. Yet his fable is quite ambiguous in its meaning, which reveals both his superior wit and his awareness that a situation's superficial appearance does not contain its entire meaning. Instead, even in a calm and basic power dynamic, there are many possibilities for power to shift. The bird might be smaller, but it has great power.
Finally, it is worth considering the historical nature of the play, which Webster used to give the play more gravity. Act 4 ends with the election of a new Pope, which was historically accurate to the time of Vittoria's controversial trial and escape. (See the "The Real Vittoria Accoramboni" for more information.) Because the event conforms to reality, the ironic use of the murderer Lodovico as guard against outside corruption is doubly effective as an attack on the church and its power-hungry hypocrites. As with everything else in the play, this most sanctified of processes is not quite what it seems, and gives us every reason to doubt its goodness.