Othello Act 4, Scene 1: " Before the citadel."
Iago tricks Othello further, causing him to believe that Desdemona is cheating on him, as evidenced by the whereabouts of the handkerchief. Othello faints upon hearing that his beloved may have lied with someone else. Iago excites himself with deception: "Work on, / My medicine, work! Thus credulous fools are caught, / And many worthy and chaste dames even thus, / All guiltless, meet reproach" Act 4, Scene 1.
Cassio enters and tries to help the unconscious Othello; Iago tells him to leave, that Othello must spend some time in his current state. Cassio exits and Othello awakens, still distraught with the information he had just received. Iago tells him that Cassio desires to speak with him. Othello plans to observe Cassio closely, in particular his actions with Desdemona.
As Othello watches, Iago plots to use Bianca in his fraudulent actions. Cassio puts Bianca down as Iago talks of her love for him. Othello is disturbed by his harsh, disheartening denial of love. Bianca enters, enraged with Cassio for giving her a handkerchief from such a minx. Othello recognizes it as the handkerchief he gave to Desdemona. Bianca storms out, followed by Cassio. Othello tells Iago of his rage, and plans to avenge his supposed cuckoldry: "Ay, let her rot and perish, and be damned tonight, for she shall not live! No, my heart is turned to stone: I strike it, and it hurts my hand" Act 4, Scene 1.
Lodovico and Desdemona come to Cyprus from Venice with great fanfare to deliver Othello a letter. Desdemona speaks to Othello of her friendship with Cassio and begs him to mend his relationship with Cassio, as well. Because of her kind words of Cassio, Othello believes the rumors of their affair and strikes Desdemona. Lodovico cannot believe such a violent act; he comments that it would never occur in Venice. Othello curses all womanhood and sends Desdemona away, as he must return to Venice. After Othello exits, Iago and Lodovico discuss the so-called benevolent Moor's actions in disbelief.