Act IV begins with the revelation of Daddy's cancer, a revelation that immediately splits the family into its respective camps. The good children, that is, the successful Gooper and fertile Mae, reveal themselves in their avarice, envy, and greed. Speaking in Big Daddy's name, Mama identifies Brick and Maggie as his rightful heirs.
The scene begins in dramatic irony, Mama still unaware of her husband's cancer. Especially poignant is how Mama marvels at how much Daddy ate at dinner. Note how Gooper looks to Daddy's certain suffering with "grim relish." When Gooper and the doctor begin to tell Mama, Mae, as always, performs a burlesque of the dutiful daughter-in-law. Her eagerness for the revelation is clear nevertheless. Mama pushes her aside.
The revelation of Daddy's cancer to Mama is the principle action of this scene. As noted above, Mama appears as a comic and touching figure, a naïve, sincere woman who does her femininity wrong in her tragically bad taste and notoriously crude manners. Devoted to a husband who has no interest in her, Mama is a woman who above all has stood by her man. The play is enamored and at the same time somewhat amused with this image of dogged feminine loyalty. The revelation of Daddy's cancer is Mama's dignified moment.
Upon the revelation, Mama reveals her investments immediately, calling for her only son and begging Maggie to help him get on his feet so he can take over the estate. Gooper and Mae spring into action, appearing at their most vicious, presenting themselves as the family's rightful heirs. Their sadism reveals itself. Gooper savors Daddy's suffering, wanting him drugged up and dead. He has always resented his parents' love for Brick and has moved to protect his interests. They present Mama with a will that she firmly rejects. As Gooper has warned, however, he knows how to protect his interests.
Oblivious to Gooper, Mama flings herself awkwardly against the indifferent Brick, his coolness forcing another woman into helpless desire. Though she knows all too well what has been going on, Mama places all hopes in Brick, in his assumption of his duties as Daddy's rightful heir. Brick must become a family man: he must provide Big Daddy with a "grandson as much like his son as his son is like Big Daddy." We have already remarked upon the narcissism of Big Daddy's dream. Though not explicitly observed, it is clear that the perpetuation of the family line through Brick is Daddy's immortality.
Brick turns from Mama, unable to comfort her, leaving Maggie to assure her that he recognizes her plea. He appears utterly removed from the travesty before him, singing to himself softly, moving in and out of the room, turning the phonograph and drowning the others out, progressively withdrawing into his drunken haze. Brick's "almost deadness" makes it impossible for him to fulfill his filial duties and assume his place in the family line.