| Quote #4
Iago's pretty good at manipulating Othello, don't you think? Here, he pretends to warn Othello not to be a jealous man, pointing out that jealousy ends up destroying the heart of the man who falls prey to it.
| Quote #5
Here, Othello claims that he won't be destroyed by jealousy. He reasons that Desdemona "had eyes, and chose [him]" despite, presumably, the fact that he is black. But, then, Othello lets slip that he may in fact be a bit more jealous and suspicious of his wife than he lets on – he says he wants some "proof" of Desdemona's infidelity. Looks like Iago's master plan may work out after all.
| Quote #6
Iago realizes that real proof of Desdemona's supposed infidelity is not necessary because mere suspicion is enough to feed Othello's jealousy. In the case of Othello, Iago will use the handkerchief Othello gave Desdemona in order to convince Othello that Desdemona's been cheating. (Remember, when Desdemona dropped her handkerchief by accident, Emilia picked it up and gave it to Iago. Iago says he's going to drop it for Cassio to find.) Even though the handkerchief is a mere "trifle, light as air," once Othello sees it in another man's possession, he'll think he has solid proof that Desdemona is unfaithful. When Iago says the handkerchief will be "as proofs of holy writ" to Othello, he means that Othello will see the handkerchief as the gospel truth that Desdemona's a cheater.