Dusk approaches in the studio. Gina has just taken a photograph of two sweethearts and stands in the doorway bidding them farewell. Hedvig enters, and they wonder why Hialmar has yet to return from his walk with Gregers.
A grim Hialmar returns. He refuses Gina's offer of dinner and announces that he will take up the studio work himself on the morrow. He pledges with disgust to never set into the garret again. Indeed, he almost wants to wring the wild duck's neck. When Hedvig shrieks in horror, he promises to leave it alone. Hialmar is taking up the claims of the ideal, the claims of his soul.
Hialmar urges Hedvig to go out. It is dark enough now, and the vapors in the house are bad for her. Once she has departed, he tells Gina he will also keep the account books himself. He asks her why she never told him his father was so liberally paid by Hakon Werle. Gina first attempts to evade his implicit accusations but ultimately demands that Hialmar explain what Gregers has told him.
Hialmar asks if Gina was Werle's concubine. Gina confesses that when Werle approached her during her service as his servant, she refused. Convinced of her guilt, Werle's wife drove her out of the house. After her death, however, the two became lovers. Hialmar is enraged by her deceit. Gina protests that he would have come to a bad end without a practical wife such as herself. Hialmar moans that he will never finish his invention. The breadwinner's dream of the well-to- do-widow who lives in the wake of his successes will come to naught.
Gregers enters, celebrating the commencement of the couple's new life, a communion founded on truth. He is shocked however by the "dullness, oppression, and gloom" in the household; the "light of transfiguration" should shine from them both. Hialmar should rejoice in forgiving his erring wife and raising her up to someone worthy of love. Gina sarcastically removes the lampshade.
Relling enters and denounces Gregers as a quack. He says that no marriage is based on the claims of the ideal. Relling reminds the Ekdals of the threat their conflict to Hedvig, and at a critical age no less. Gina notes her changing temperament and sees that Hedvig has taken to playing house-on-fire with the stove in the kitchen.
Suddenly Mrs. Sorby appears at the door. She has come to bid farewell to Gina as she and Werle are moving to the Hoidal works tomorrow. Gregers reveals their marriage plans. Relling bitterly wishes her happiness. Apparently Relling and Sorby once shared a liaison. When Gregers threatens to tell his father of their affair, Mrs. Sorby informs him that they have confessed all their secrets to each other and theirs is an honest marriage.
Sorby has pledged to care for Werle in his increasing blindness. Hialmar is shocked that Werle is losing his eyesight too. As she moves to leave, Sorby instructs Hialmar to apply to Graberg if he needs anything. Shocking even Gregers, Hialmar not only declines this offer of help but promises to pay back their longstanding debt to the firm—a debt far beyond the Ekdals' means.
Act IV begins at dusk with Gina bidding farewell to two sweethearts she has just photographed. This heavy-handedly symbolic departure prefigures the demystification of, to recall Gregers' phrase, the Ekdals' "tableau of familial affection." In learning Gina's past and the extent of his family's dependence on Werle, Hialmar can no longer sustain his fantasies of himself as breadwinner and father. Much to Gregers's disbelief, this demystification does not bring illumination to the household but its opposite.
Once again, light/dark oppositions structure the dialogue. As Gregers observes, the "light of transfiguration" fails to shine forth from the couple after their confrontation. Instead, "dullness, oppression, and gloom"—perhaps reminiscent of the smoke Gregers produced in the stove fire—pervades the household. As noted above, Gregers expects such light to fill the household as he pathologically imagines Hialmar as a "shining light" among men. He suffers, as Relling will argue later, a disease of hero-worship and romantic idealization. Gregers's fantasies of idolatry underpin his logic of redemption. In finally becoming himself, Hialmar would attain the ideal and then forgive his fallen wife by raising her to his own exalted level. The ideal would save them all.
Hialmar all too readily parrots his friend's gospel, returning from their walk completely converted to the claim of the ideal. In this act especially does his speech ring heady and bombastic, petty and cruel. Note, for example, his appeal to the "breadwinner's dream" of providing for a well-to-do-widow. Hialmar's hyperbole is clearly aggressive in intent. He conjures his death to torment the wife who has "betrayed" him.
In response to Gregers and her husband's language of light/dark, spiritual tumults, taints, poisons, and onward, Gina again emphasizes "the practical." Without such a practical wife, the fiery Hialmar would have been far worse off. In particular, Gina delivers one of the more memorable jokes of the play in removing the lampshade in response to Gregers' exhortations. The joke operates by shifting from the plane of Salvationist allegory to physical comedy, from Gregers' impassioned rhetoric to the banal household object. The removal of the lampshade recalls how the petty concerns of the household function throughout the play to deflate the tirades of our would-be romantic heroes.
If the pathos of these would-be heroes often falls flat, the play continually makes use of the at times cheap pathos of the martyr, specifically the martyred child and the old man. In contrast to the pathetic Hialmar, Hedvig and Ekdal's suffering continuously appears pitiable. They are wounded ducks, shot down by hunters or torn from their families. Here Hedvig in particular functions as an object of pity: whatever happens between Gina and Hialmar, they must protect the child.
Finally, in Relling's exhortations, the child, and specifically the daughter, become an object of quasi-medical inquiry. On the cusp of a birthday, Hedvig is at a particularly critical junction in her adolescence and her constitution is changing. Her irrationality manifests itself in her games of house-on-fire. The significance of this game remains unclear though the weighty symbolism throughout the play invites speculation.