In Act IV, we have returned to the Chiltern's residence. It is the next morning and Lord Goring is waiting in the morning room. To his dismay, his father shows up, taking the opportunity to renew his criticisms of his son before excitedly telling Arthur what happened in the House of Commons: Sir Robert made a speech harshly denouncing the canal scheme as a swindle. Mabel then enters the room, ignoring Arthur because he didn't show up for their riding date and speaking only to his father. When Lord Caversham leaves the room, Lord Goring asks Mabel to marry him. Mabel responds flippantly, and ironically, the normally frivolous Arthur is reduced to imploring her to be serious. Mabel admits her love for him and the two embrace. At this point Gertrude enters the room, and since Lord Goring needs to talk to her, Mabel goes to the conservatory to wait for him.
Arthur informs Gertrude that he obtained Sir Robert's letter from Mrs. Cheveley and has burned it. She's relieved that her husband won't be exposed, but then Arthur has to tell her the bad news: Mrs. Cheveley stole Gertrude's note to him and is planning to send it to Sir Robert to try to cause trouble. Gertrude explains that she changed her mind about coming to see him, and Arthur advises her to tell Robert the truth and thereby disarm Mrs. Cheveley. Horrified, Gertrude says that she can't possibly tell her husband that she was planning to visit a bachelor in his rooms, no matter how innocently, and tells Arthur that they must somehow intercept the letter. Unfortunately, Sir Robert has already received and read the note and comes in search of his wife. However, since Gertrude hadn't actually addressed the note to Lord Goring, and Mrs. Cheveley sent it anonymously, Robert assumes that Gertrude was writing to him, forgiving him. He has come in search of her, filled with love and gratitude. Meeting Arthur's pleading eyes, Gertrude does nothing to correct this misapprehension. Figuring that they could use some privacy to reconcile, Goring goes to join Mabel in the conservatory.
Gertrude informs Robert that Lord Goring obtained and destroyed the incriminating letter; his reputation is safe. He is greatly relieved, and hesitantly offers to now retire from public life, obviously hoping that Gertrude won't think it's a good idea. To his dismay, though, Gertrude approves of this idea, thinking that this is the honourable thing to do. Lord Goring comes back in, and Sir Robert gratefully thanks him for his aid. Arthur is going to seize the moment to get Robert's permission to marry Mabel, but his father returns and interrupts him. He congratulates Sir Robert on his speech in the House, and tells him that, impressed by his "high moral tone" the Prime Minister is going to offer him a seat in the Cabinet. Sir Robert is excited by this but, remembering his promise to Gertrude, reluctantly tells the incredulous Lord Caversham that he'll have to decline. Gertrude admires his decision to do the right thing and warmly tells him that she'll help him write the letter to the Prime Minister refusing the position. They leave the room. Lord Caversham thinks they're both crazy, but Arthur tells him that the decision demonstrates Sir Robert's "high moral tone". Unconvinced, Lord Caversham says that in his day they called it idiocy. Needing his father out of the way for a few minutes, Arthur sends him into the conservatory to keep Mabel company.
When Gertrude reenters the room, Arthur asks her why she's giving Mrs. Cheveley what she wanted, having Sir Robert destroy his own career. Gertrude says that Robert decided himself that it was the right thing to do, but Arthur tells her that Robert is doing it because he fears losing her love. He also says that Sir Robert was made for public life and will never be truly happy away from it. He predicts that, if she forces Robert to go through with this, both he and Gertrude will end up regretting it bitterly. Sir Robert comes into the room carrying his letter to the Prime Minister. Gertrude takes it from him and rips it up, realizing that Arthur is right.
Gertrude and Sir Robert embrace, truly reconciled. Taking advantage of the moment, Lord Goring asks Sir Robert for Mabel's hand in marriage. Though he is grateful to Arthur for all his help, Robert tells him that he must refuse. Shocked, Arthur asks for an explanation and Sir Robert tells him that, after finding Mrs. Cheveley at his house at night, and after Arthur's impassioned defense of her, he realizes that his friend still has some sort of connection to his former fiance. Sir Robert says that he can't allow his sister to marry a man who clearly has feelings for another woman. Lord Goring cannot defend himself without betraying Gertrude's confidence, and so stays silent. Gertrude, however, admits that it was she whom Arthur was expecting to be in the drawing room, and that the note which Robert thought was for him was actually for Goring. Robert tells his wife that he would never have believed any ill of her, and Gertrude takes the note and writes Sir Robert's name at the top, saying that it is he that she trusts and needs. Mabel and Lord Caversham come in from the conservatory, and Arthur's engagement to her is announces. His father warns Arthur that he'd better be an ideal husband to her, but Mabel says that she wouldn't want her husband to be ideal; she would prefer that Arthur be himself. Everyone leaves the room except for Sir Robert, who sits by himself, brooding. When Gertrude comes back to find him, he asks her if she truly loves him, or merely feels sorry for him. Gertrude assures him that her love for him is real and abiding, and they begin their married life anew, on a more understanding and healthy footing.