Diomedes comes to Troy to make the exchange of Antenor for Cressida, and he is greeted heartily by Aeneas and Paris. Aeneas goes to fetch Cressida, remarking that this exchange will deal a heavy blow to Troilus; Paris concurs, but says regretfully that they have no choice: "the bitter disposition of the time / will have it so" (IV.i.48-49). After Aeneas is gone, Diomedes is asked who he thinks deserves Helen more—Paris or Menelaus? With great bitterness, the Greek replies that both deserve her, since both are fools, willing to pay a great price in blood for a "whore."
Meanwhile, as morning breaks, Troilus takes a regretful leave of Cressida while she pleas with him to stay a little longer. Pandarus comes in and makes several bawdy jokes about their recent lovemaking; suddenly, there is a knock at the door, and Cressida hides Troilus in her bedroom. Aeneas enters, and demands that Pandarus bring out Troilus. When the young prince emerges, Aeneas gives him the heavy news that Cressida must be sent to her father in the Greek camp. Troilus is distraught, and goes with Aeneas to see his father, Priam, while Pandarus breaks the news to Cressida, who begins to weep.
Troilus brings Diomedes, together with the great lords of Troy, to Cressida's house, and begs leave to say goodbye to his lady. When they are alone, he pledges to be faithful, and Cressida promises that even in the Greek camp, she will remain true to him. Then Diomedes is brought in, and Troilus demands that he "use her well...for, by the dreadful Pluto, if thou dost not, / Though the great bulk Achilles be thy guard, / I'll cut thy throat" (IV.iv.124-9). Diomedes retorts that he will make no promises—he will treat Cressida as she deserves, but not because any Trojan prince orders him to. At that moment, a trumpet sounds, calling them all to the Greek camp for the duel between Hector and Ajax.
The theme of lust, not romantic love, as the true motivation at Troy is emphasized in these scenes. First, we have Diomedes debunking Helen, whose great beauty provided the occasion for the war. In the debate with his brothers, Troilus called her "a theme of honor and renown / A spur to valiant and magnanimous deeds" (II.ii.199- 200). But in the context of the play, Diomedes's analysis of the conflict seems more appropriate:
He merits well enough to have her that doth seek her, Not making any scruple of her soilure... He, like a puling cuckold, would drink up The lees and dregs of a flat tamed piece, You, like a lecher, out of whorish loins Are pleased to breed out your inheritors. Both merits poised, each weighs nor less nor more; But he as he, the heavier for a whore. (IV.i.55-66)
In the context of the epic tradition, this is Shakespeare's ultimate act of debunking the epic stories of classical Greece. Not only are the heroes to be unheroic, violent, lustful fools, but Helen, the great beauty, is described by Diomedes as a "whore" being fought over by a "cuckold" and a "lecher."
With Helen thus described as a Spartan whore, it is easy enough to see Cressida as her Trojan counterpart. Troilus and Cressida's morning parting scene reminds one, again, of Romeo and Juliet—save that the couple is again interrupted by the cheery voyeur Pandarus, who comes prancing in saying "How now, how now! How go maidenheads?" (IV.ii.23) and then, "Hast not slept tonight? Would he not, a naughty man, let it sleep?" (IV.ii.31-32). Any profound resonance contained in their parting—which will soon become permanent, because the affairs of state are about to intervene in their romance—leaks out with the arrival of the bawdy uncle, whose language reduces Troilus and Cressida to a pair of highly sexed adolescents.
The scene that follows between Diomedes and Troilus is almost all foreshadowing: we have Troilus making a futile plea to the Greek to "use her well," which carries a heavy as yet unrealized double meaning, and then Diomedes's retort that "I'll answer to my lust" (IV.iv.132), which in Shakespearean English means "I'll do as I please," but which also carries another obvious implication. That implication, as the audience well knows, will soon be realized.
I just finished reading Troilus and Cressida in my effort to read all Shakespeare by his 450th birthday. It wasn't a favorite play, and I probably could have had a very happy life without ever reading it. But in case you're interested, here's my take: