In the second scene of Act 4, Belvile engages in a duel against Pedro on behalf of Antonio. Because the men are disguised, their identities are concealed from each other. This scene is filled with dramatic irony, as those participating and those observing the duel have varying information about the identities of the two combatants—only the audience has all the information.
Pedro believes he's dueling Antonio in defense of his sister. Belvile doesn't know who he's dueling but believes it is yet another rival for Florinda's heart. Florinda, watching the duel, initially fears that Pedro is fighting Belvile but hears Pedro mention Antonio's name. She finally feels at ease when Belvile enters dressed as Antonio. This is ironic, of course, because Belvile is fighting against her brother, right in front of her eyes.
The scene encapsulates all the confusion and misunderstanding that arises from the play's recurrent masking and disguise. As a result of Carnival, the characters' identities are repeatedly mistaken. Nothing is as it seems—some don't know against whom they are fighting, others don't know on whose side they are.
The ultimate irony of the scene is that Belvile puts himself in danger to defend his rival for Florinda's hand. The audience recognizes this irony, as Belvile's sole quest until this point has been a reunion with Florinda and the opportunity to express his love to her. When Belvile comes to be indebted to Antonio, who knows nothing about the relationship between Belvile and Florinda, he has no choice but to help him in a duel over the woman he loves. He resolves to go through with it because he believes he will be fighting against another rival for Florinda's heart and hand, when he is in fact fighting against Florinda's brother to prove Antonio's respect for and devotion to her.
At the end of the duel, Pedro declares that Antonio has redeemed himself and that he can marry Florinda after all:
PEDRO Antonio, you’ve done enough to prove you love Florinda.
BELVILE Love Florinda! Does Heaven love adoration! prayer! or penitence! Love her! here, sir, – your sword again. [Snatches up the sword and gives it him. Upon this truth I’ll fight my life away.
PEDRO No, you’ve redeemed my sister, and my friendship!
Pedro, who still thinks he has fought against Antonio, is impressed by his opponent's brave duel in defense of his sister. Because Belvile thought he was fighting against another rival for Florinda, Pedro's words surprise him. He says he is willing to fight his "life away" to prove "this truth." Pedro declares that Antonio has done more than enough to both redeem his sister and his friendship. It thus turns out that by fighting bravely in the duel, Belvile has reinforced his only rival's claim over Florinda.
Once Belvile's identity is revealed by the unhelpful Willmore, Pedro takes his words back. He concedes that Belvile fought bravely, but that "’Twas by Antonio’s, not by Belvile’s sword" that he fought for his right to marry Florinda. After speaking this line, Pedro exits with Florinda and Belvile is as far as he's ever been from being reunited with her.
