The fourth act begins with a conversation between Iago and Othello. The audience finds them in the middle of a conversation in which they are discussing the possible affair between Cassio and Desdemona. Iago uses a metaphor of a couple in bed together naked, but not having sex (lines 3-4). The metaphor suggests that Desdemona and Cassio may not yet have had sexual relations, but they appear to be very close to doing so. In this metaphor Iago states that lying together but not having sex is harmless. Iago is, of course, being quite crafty: he is planting an image of Cassio and Desdemona lying together naked. And such an image is quite upsetting to Othello.
Othello describes Iago’s image as “hypocrisy against the devil” (6). Othello means that the devil himself would view such a naked man and woman in bed together as sinners whether they committed the sexual act or not. In other words, Othello is stating his own belief that Desdemona is guilty of having an affair even if she has not yet had sex with Cassio. Just the intention or desire to have sex makes her guilty. Referring back to the metaphorical couple, Othello adds that “they tempt heaven” (8). Othello suggests that possibly even God would find such an act to be sinful.
Iago now subtly reminds Othello about the handkerchief (line 10). This is the proof or evidence that convinces Othello about Desdemona’s infidelity.
Being reminded of it, right after imagining Desdemona lying naked with Cassio, disturbs and upsets Othello even more. This is an intensely emotional scene. The actor playing Othello must show the anger and hurt growing inside him. Perhaps his body would quiver, and his voice would waver and rise in pitch and volume.
Iago (again in his subtle manner to anger and madden Othello) tells Othello that a person who receives a gift (like a handkerchief) has the right to give it away. But Othello responds that such a person does not have the right to give away her honor (line 14). Iago repeats his own belief that honor is meaningless: “They have it very oft that have it not” (16). Iago means that many people who are believed to be honorable are actually quite dishonorable. Iago then steers Othello’s thoughts back onto the handkerchief. Iago can visibly see the effect of his words on Othello. Iago knows how Othello will think concerning the handkerchief. The handkerchief symbolizes Desdemona’s honor. So, in Othello’s mind, when Desdemona gives the handkerchief away, she is also giving her honor away.
Othello no longer thinks that Desdemona is honorable. Othello is on the edge. He is emotionally unstable. And now he is ready for Iago to push him off the edge. And Iago does so. Iago tells Othello that Cassio told him that he is having sex with Desdemona (line 33). The image that forms in his mind is too hard for Othello to bear. He is driven over the edge. The thought drives him crazy. The reader should note how Othello’s dialogue has suddenly reverted to prose (lines 34-41). And
Othello’s own words become nearly incomprehensible. He is picturing the affair in his mind, and the thought is too hard for him to take. So, Othello falls into a trance. He can no longer connect with reality. He can no longer see or hear what is happening around him. He is locked in his own wild mind. Othello, at least for the moment, has gone mad.
Iago’s plan is working just as he had hoped, but he is not finished yet. Othello awakens quickly from his trance, and this perhaps suggests that he has not completely lost all of his mental abilities. Iago realizes that Othello still needs to be pushed or manipulated further so that he will not change his mind about believing that Desdemona is unfaithful. When Othello awakens from his trance, he puts his hand on his head. Iago asks him, “Have you not hurt your head?” (56). This line has a literal meaning, but also a symbolic one. According to medieval superstition, a cuckold (a man whose wife is having an affair) grows horns upon his forehead. Iago indirectly is calling Othello a cuckold. And, in Shakespeare’s day, a cuckold is also a fool.
Othello still wants direct and substantial proof that Desdemona is having an affair. Iago tells Othello that he can get such proof directly from Cassio’s own lips. Iago instructs Othello to hide behind a wall or some other object and secretly listen while he talks to Cassio. Othello does as Iago asks. Othello eavesdrops on the conversation that follows (lines 102-62).
Iago, of course, knows that Cassio is not having an affair with Desdemona. But Iago also knows that Cassio is having a relationship with a commoner named Bianca. Cassio does not view this relationship seriously. Bianca is someone he likes to have fun with, but he certainly has no intention to marry her. Cassio has jokingly talked about Bianca to Iago in the past (lines 96-97). Iago’s intention is to get Cassio to speak jokingly of Bianca again, but to make Othello (who is hiding at a little distance away) think that Cassio is talking about Desdemona.
The conversation works because Iago, when he mentions Bianca’s name, says it in a whisper so that Othello does not hear it. Out loud, so Othello can hear, Iago tells Cassio to “ply Desdemona well and you are sure on’t” (104). The word ply here means to petition or ask. Iago is telling Cassio to keep asking Desdemona for assistance in order for him to get back his job as lieutenant. Then, softly, in a whisper, Iago adds, “Now, if this suit lay in Bianca’s power, how quickly you should speed” (105-06). The word speed means to be successful. Iago is saying that Cassio’s suit or request to get his job back would be easy and successful if he were asking Bianca instead of Desdemona. Bianca would do anything for Cassio because she is hopelessly and foolishly in love with him. What is important here is that Othello does not hear these two lines. So, when Cassio starts laughing about Bianca, Othello believes that he is laughing about Desdemona. And, as Cassio continues to joke about Bianca, Othello comes to believe the worst about Cassio. Othello believes that Cassio is a coldhearted lover who is just having a casual sexual relationship with Desdemona and then intends to abandon her.
The case against Cassio looks even worse when Bianca appears and hands the handkerchief back to Cassio (lines 140-50). Bianca is angry that Cassio asked her to make a copy of the embroidered handkerchief. She thinks that the handkerchief is a love token that some other woman gave to Cassio. Bianca is jealous, and her words convey the impression that Cassio is a cold and heartless lover who chases after many women.
Othello is now convinced. After Cassio exits, Othello asks, “How shall I murder him, Iago?” (163).
Othello wants revenge.