Act 4

http://www.youtube.com/watch?v=sVknfdGRS5I
http://www.youtube.com/watch?v=wHVnLyP9ZVE

Iago continues his insinuations when speaking to Othello; he provides more "proofs" that are anything but ocular, though Othello has calmed, and seems more troubled and less angry. In the last act Othello was trying to act as Desdemona's defender, and Iago was the accuser; ironically, they seem to have switched places here, and Iago seems to be defending Desdemona, all while producing more "evidence" to condemn her. The handkerchief, however, is as important a symbol as ever; "it comes o'er my memory as a raven doth over an infected house," Othello says. The metaphor highlights how crucial this object is to him, as Othello burdens that single object with more and more significance.
Note the contrast between Othello's language as he falls into a trance, and Othello's language in any previous part of the play, including Act III. He speaks in single, disconnected words‹"handkerchief‹confession‹handkerchief," or "Noses, ears, and lips"‹that completely belie his ability to speak coherently and elegantly, as Othello has shown, especially in Act I with Brabantio. The lack of connection in his language parallels his descent into emotional and logical chaos; as he becomes more upset, without a true cause, he falls farther and farther from himself, and the order which typically rules him. Again, the theme of order vs. chaos comes into play.
Othello's trance also marks his descent into the savage; ironically, he becomes the passion-stirred, wicked pagan that others had accused him of being, merely because of his skin color. Iago notes that Othello "breaks out into savage madness" in this fit; indeed, the primal seems to be taking over the more civilized aspects of Othello. Othello refers to himself as a "horned man," ashamed of this descent; yet it has settled upon him, and he will struggle in vain to regain his dignity.
"O, the world hath not a sweeter creature," Othello declares of Desdemona; yet, against his reason and better nature, he decides that she shall not live for what she has supposedly done. There is great irony in this scene, as Othello declares that Desdemona is of a soft and kind nature, yet condemns her for being lustful and immoral. Note Othello's reticent tone, even when he is condemning Desdemona to death; although chaos and jealousy have triumphed over reason, still there is a part of him that knows Desdemona is good, and does not want to condemn her.
When Othello strikes Desdemona, he shows the severity of his change. Just her mention of Cassio sends him into an unreasonable rage; every little thing he regards with suspicion, even if he has no cause. Although one of his greatest fears regarding Desdemona's alleged infidelity was that it would blacken his name and reputation, the irony is that Othello is doing that himself; in striking Othello and being unreasonably cruel, he besmirches his own good name. Savagery is taking over his civility, as he continues to become the cruel, jealous, passion-spurred "savage" that Brabantio accused him of being.