| Quote #4
Iago is completely obsessed with infidelity. Earlier, we heard him say that he suspects Othello has slept with Emilia (a sentiment he repeats in this passage). Not only that, he also says he "fear[s]" that even Cassio is sleeping with his wife. What's Iago going to do about it? Why, he's going to try to sleep with Desdemona, which will allow him to get even with the "lusty Moor." If he can't do that, he wants to make Othello believe that Desdemona is screwing around with Cassio.
| Quote #5
When Iago wants to make Othello suspect Desdemona's been unfaithful, he suggests a woman who disobeys and "deceive[s] her father is likely to screw around on her husband. Othello's response implies that he feels the same way. Instead of seeing Desdemona's decision to elope with Othello (despite her father's disapproval) as a sign of his wife's loyalty to him, Othello sees Desdemona's willingness to elope as a prelude to her infidelity. It seems that Othello's sexist assumptions leave him pretty vulnerable to Iago's plotting.
| Quote #6
Get your highlighter out because this is important. When Othello is convinced (by Iago) that Desdemona has cheated on him, he reveals something pretty interesting about himself. It seems that Othello believes all men, both "great" and "base," are "destin[ed]" to be cuckolds. FYI: A "cuckold" is a man whose been cheated on by his wife – cuckolds are commonly associated with horns, which is why Othello refers to cuckoldry as a "forked plague" that men suffer from.
So, if Othello believes that all men are destined, from the moment of their birth, to be cheated on by their wives, then this helps to explain why Othello is so easily convinced that Desdemona has been unfaithful, despite the fact that Iago never actually shows Othello any real evidence.