Act IV begins with Barabas congratulating himself on the success of his plan to poison the nuns. Ithamore fears that they will be caught, but Barabas reminds him that only the two of them know they are to blame for the deed. The merchant says that he will cut the slave's throat if he confesses to anyone. Barabas admits that he is not upset at Abigail's death, although he still despairs that she converted to Christianity. The friars Jacomo and Bernardine enter and clumsily try to make Barabas confess his role in the deaths of Mathias and Lodowick. The dialogue between the three men is rather comic as Barabas keeps interrupting the stammering priests only to confess to irrelevant crimes. For example, when Bernardine states, "Thou hast committed," Barabas interrupts with "Fornication? / But that was in another country: / And besides, the wench is dead." When the merchant realizes that his plot against the young men has been uncovered he changes tack and starts to "dissemble." Barabas assures the friars that he wants to convert to Christianity and atone for his sins, slyly promising that he will donate his wealth to whichever religious denomination he joins. Of course, the two priests start trying to persuade the protagonist to join each's own monastery. They denounce the strict rules of each other's groups. Barabas assures Bernardine that he will join his monastery, but, after the friar has left with Ithamore, the merchant promises Jacomo that he will join the Dominicans. Jacomo leaves after swearing that he will not tell anyone in his convent about Barabas's decision.
Barabas then decides to murder Jacomo in retribution for Abigail's conversion, and to kill Bernardine because he knows too much. Ithamore tells his master that Bernardine is asleep and that there is no way he can escape from the merchant's house. Bernadine makes Ithamore strangle the priest with his own belt, and the two murderers leave the corpse propped up on his staff outside in the street. Jacomo appears, sees Bernardine, and deduces that the friar was lying in wait to intercept him. Jacomo hits Bernardine over the head with a staff, not realizing that Bernardine is already dead. Barabas and Ithamore suddenly appear and accuse Jacomo of killing the friar. Barabas pretends that he wanted to join Jacomo's monastery but states he will not do so given the bad example the friar has set. Barabas says he cannot let the terrified Jacomo escape because the "law must have his course." The men leave to hand the priest over to officials.
Pilia-Borza returns from delivering a letter to Ithamore from Bellamira. The pimp tells Bellamira that he met the slave at the gallows, where Ithamore was watching Jacomo's execution. It will emerge that Pilia-Borza and Bellamira plan to use Ithamore to get to Barabas's money. Ithamore then arrives at Bellamira's house, having been instructed to go there in the letter. He is totally bewildered by the respect the courtesan and the pimp show toward him. Bellamira professes her love for Ithamore, and Ithamore tells himself that he should leave and get money from Barabas to improve his appearance. The prostitute encourages Ithamore to stay and beguiles him into bribing his master. The slave writes a letter to Barabas, demanding 300 crowns or else he will confess about their crimes. While all this is happening, Bellamira and Pilia- Borza make disparaging remarks about Ithamore in asides to each other. The pimp leaves to deliver the ultimatum to Barabas. The prostitute promises to marry the slave, at which point he breaks off into poetic rhapsodies about their future together. Pilia-Borza returns and says that the merchant "laughed and jeered" about Ithamore's loyalty and only gave the pimp ten crowns. The slave writes another letter to Barabas, requesting 500 crowns for himself and 100 for Pilia- Borza. The pimp leaves, and Bellamira throws away the ten crowns Ithamore has offered her in an attempt to convince him of her (false) love. She kisses the slave before inviting him to sleep with her.
Duplicity and cunning play a central role in the first and second scenes of Act IV. Barabas attains new heights of iniquity as he manages to kill both priests, leaving his role in Mathias's and Lodowick's deaths undisclosed. Barabas also has some sport with the friars as he enjoys watching them insult one another in a futile attempt to gain his wealth. As in previous scenes, such as Act III, scene vi, the clergymen come across as highly flawed individuals who care more about earthly riches than heavenly concerns. Interestingly, Marlowe chooses to have Barabas call the friars' weakness and inherent vice to our attention. When Barabas witnesses the priests' brawl he does not step in to separate them, as Ithamore suggests, but pronounces instead, "This is mere frailty, brethren, be content." However, Barabas's jocularity is deadly rather than good-humored and he shows no remorse about letting Jacomo hang for a murder the friar did not commit. Clearly, it is the protagonist's intelligence as well as his sheer cunning that lets him get away with his acts. The merchant displays his legal knowledge when he notes to Ithamore that they must show officials the staff used by Jacomo to hit Bernardine, since "[l]aw wills that each particular be shown." Coming after Barabas's successful plots to gain revenge on Ferneze and poison the nuns, the completion of his new scheme underscores his genius as a criminal mastermind. This great villain delights in plotting for its own sake, since the gains he achieves through crime are merely a bonus. Barabas thus displays Machiavellian principles applied to the extreme. Rather than the end justifying the means, as Machiavelli holds, the means become an end in themselves as the criminal revels in his own villainy.
Although less malevolent than Barabas, Bellamira and Pilia-Borza show themselves to be similarly unscrupulous in their manipulation of Ithamore. Although the slave is not an innocent character—in fact, as a murderer, he is more cunning than many people he encounters—he is out of his depth when confronted by Bellamira's wily seduction. He even seems naïve on occasion, particularly when he rhapsodizes about what his future will be like with the courtesan. In a fine example of romantic self-delusion, Ithamore assures Bellamira, "Where woods and forests go in goodly green, / I'll be Adonis, thou shalt be Love's Queen." However, while he professes to love the courtesan, Marlowe suggests that Ithamore lusts after money. He is perhaps too quick to jump at the chance of bribing Barabas; Ithamore's loyalty vanishes in an instant as he realizes he can coerce his master for gold. Truth has a price—and Barabas must pay to keep it undisclosed. Marlowe shows how those who are willing to trade on truth may profit.