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.
The Chilterns share their own moment of bliss, and Lord Goring finally asks for Mabel's hand in marriage. On account of his discovery of Mrs. Cheveley at Goring's last night, however, Sir Robert cannot consent; Goring is left silent. Dramatically, Lady Chiltern thus reveals that Lord Goring expected her in his drawing room last night and that she authored the pink note as a request for his assistance. All reconcile, and Lady Chiltern writes Sir Robert's name at the top of her "love letter."
Mabel and Caversham then enter and, much to the latter's surprise, she announces her engagement with Goring. Caversham commands his son to be for Mabel an ideal husband at the risk of disinheritance; jokingly, Mabel recoils. "An ideal husband!" she exclaims. "Oh, I don't think I should like that. It sounds like something in the next world." Goring can be what he likes; Mabel, overcome, only wants to be a "real wife" to him.
All exit except for Sir Robert, who sits alone pensively as at the end of Act I. Lady Chiltern returns, and he asks if she loves or merely pities him. Lady Chiltern pledges her love and the beginning of a new life for them both.
As with the popular domestic comedies upon which An Ideal Husband is based, Act IV brings us to a culminating restoration of married life. All is set right: Sir Robert preserves his public image and indeed even advances in his career; the Chilterns' reunite; the young lovers, Goring and Mabel, come together as well. Rather than adhere strictly to a model of rising, climatic, and falling action, the act concludes the play with a series of dizzying complications—a misread note, a complex choreography of entrances, exits, and private conversations, confessions—that only resolve themselves at the very end.
As we recall from Act III, Lady Chiltern naïvely writes her note to Goring as a plea for help: "I want you. I trust you. I am coming to you." Notably, Mrs. Cheveley mocks this pink note as resembling the start of some "middle class romance," suggesting Wilde's self-irony regarding his use of this stock device. Ultimately it comes to serve as a sort of second marriage certificate, symbolizing, with the inscription of Sir Robert's name, a restoration of the Chilterns' married life. Though Goring jokingly moves to reclaim the letter, it is clearly no longer meant for him. This restoration of the conjugal household occurs on rather conventional moral terms. Once again, the language of melodrama intervenes: there is a profusion of exclamations, sighs, and somewhat trite appeals to faith, love, charity, devotion, and onward as characters succumb to emotion. Lord Goring especially delivers these pronouncements on conjugal bliss. In particular, he makes a rousing speech to Lady Chiltern upon Sir Robert's decision to withdraw from public life that establishes the proper roles of man and woman in married life. We will sketch it briefly here.
First, Goring argues that men and women alike are not worthy of sacrifices as terrible as the one Sir Robert faces. Lady Chiltern cannot allow Sir Robert to resign from public life especially, however, because Sir Robert is a man. Man's life remains of wider scope, deeper issues, and greater ambitions than woman's. Whereas a woman's life revolves in "curves of emotions," man's progresses in "lines of intellect." Consequently, women are not meant to judge men but to forgive them ("Pardon, not punishment, is their mission"). Thus Lady Chiltern must assume the role that defines Victorian womanhood in its most conventional form: that of a forgiving and anodyne caregiver.
Moreover, by demanding that Sir Robert exit public life, Lady Chiltern, according to Lord Goring, "[plays] Mrs. Cheveley's cards"—that is, she plays the part of the villainess rather than that of the heroine. What Goring means precisely by this accusation is somewhat unclear. Is the supposed fault she shares with Mrs. Cheveley is her use of love to bend her husband's will? In any case, Goring's speech leaves the audience with firmly established gender roles in the marital household that, to a contemporary reader especially, are quite disappointing. As noted above, Lady Chiltern will repeat his speech to Sir Robert verbatim, indicating that she has learned her lesson well.
At the same time, as with the entire play, Act IV offers a critique of marriage that undermines this sentimental resolution. More precisely, Goring and Mabel's marriage serves as a sort of foil to the Chilterns'. As Mabel declares in one of the penultimate moments of the play, the "ideal husband" belongs to the next world; in their marriage, Goring can be whatever he wants. She, on the other hand, promises to be a "real wife."
Thus Mabel and Goring negotiate a union that dispenses with question regarding the ideal behavior of the married couple. Indeed, throughout the play they have assumed an amoral pose, disparaging the demands of duty and respectability. Earlier in the act, for example, Mabel remarks to Goring how "on principle," she never does her duty; it always depresses her. She thus teases the lord with what one might describe as a "false paradox"—that is, a statement that is taken or misunderstood as amusingly paradoxical even as the terms involved ("duty" and "principle") are not necessarily contradictory. Read literally, Mabel's witticism suggests that the principles of these lovers demand precisely that they resist the notion of duty. Clearly then does Mabel end up on different footing than her sister-in-law, who has finally come to learn her duties to her husband.