Act IV returns us to Sir Robert's morning room with Lord Goring standing alone and looking bored. He rings the bell, and the footman informs him of his friends' whereabouts: Lady Chiltern has yet to leave the room, Mabel has returned from riding, and Lord Caversham has been waiting for Sir Robert in the library. Caversham emerges and raises the question of marriage anew, encouraging him to propose to Mabel even as he remains certain she will refuse. He also reveals that Sir Robert has roundly denounced the Argentine Canal scheme in the House of Commons, the press marking it as a turning point in his career.

Mabel then appears and, ignoring Lord Goring for his failure to make their riding appointment, banters with Caversham at his expense. Upon Caversham's departure, Goring asks her hand in marriage. Notably, Goring's proposal shows him overwhelmed, imploring Mabel to be serious and revealing his fear of her refusal. Having made Goring beg, Mabel exclaims that all of London knows of her adoration for him and accepts; the two embrace and share a moment of bliss.

Lady Chiltern then enters, and Mabel flees to the conservatory. Alone with Gertrude, Goring informs her of last night's events and warns her of Mrs. Cheveley's scheme regarding her pink letter. Once again Goring urges his listener to tell their spouse the truth; Lady Chiltern refuses and demands that the letter be intercepted.

Just as they devise a scheme to waylay the letter, Sir Robert climbs up the stairs, note in hand. He misreads it entirely, believing it addressed to him. At Lord Goring's silent plea, Lady Chiltern accepts her husband's error; Goring passes into the conservatory. She then informs Sir Robert that the infamous letter has been destroyed, safeguarding his reputation forever. With great anxiety, Sir Robert haltingly proposes that he now retire from public life, and, to his dismay, Lady Chiltern eagerly lends her support to this moral sacrifice. Goring returns from the conservatory, reconciles with Sir Robert, and is about to ask for Mabel's hand when Lord Caversham comes on the scene.

Caversham thoroughly congratulates Sir Robert on his recent speech against the canal scheme and informs him that the Prime Minister has offered him the Cabinet seat Caversham has just vacated. In other words, Sir Robert, with what Caversham describes as his "high moral tone," is the successor Goring could never be. Having just promised an early retirement to his wife, however, Sir Robert sadly declines, and Lady Chiltern glowingly urges him to write the Prime Minister immediately with his regrets. The two leave the room together. Goring sends his disapprovingly bewildered father into the conservatory, explaining that Sir Robert's decision exemplifies his "high moral tone." Comically, Caversham balks at this "newfangled" phrase, noting that in his day it was simply called "idiocy."

Lady Chiltern returns to the morning room. Goring then accuses her of "playing Mrs. Cheveley's cards," thrusting her husband from public life just when they have saved him from public dishonor. He goes on to deliver an extended speech on the appropriate roles of man and wife to dissuade her. Sir Robert then emerges from off-stage and presents his letter; Lady Chiltern takes it and tears it up, repeating Goring's words. She has learned her lesson.