It is midnight at Higgins' house. Eliza, Higgins, and Pickering all enter, tired and dressed formally. Eliza is quiet, as Higgins and Pickering recount their day: a garden party, followed by a dinner party, followed by the opera. Pickering says that Higgins has won his bet, as "Eliza did the trick."
A far cry from her behavior in Act Three, Eliza successfully passed as a noble lady at the garden party, dinner party, and opera, winning Higgins' bet for him. Though its worth noting that in winning the bet for him, Eliza's success in transforming becomes smaller than Higgins' success in transforming her.
Higgins says he knew Eliza would be fine, and tells Pickering that he has long been bored with the experiment, after its early phase. Having to go to high society events with Eliza has been irritating for him. He thanks God the experiment is over. Pickering says that Eliza was acting better than some actual noble people, who assumed that "style comes by nature to people in their position," and so didn't bother learning proper behavior.
Eliza apparently easily fooled people into thinking she was upper-class. Higgins is insensitive to Eliza's feelings, saying that he has been bored with "the experiment"—he doesn't even think of her as a person. Pickering notes that many noble people assume that they innately have proper manners, when in reality they don't because such manners must be learned (as Eliza has learned them).
Preparing for bed, Higgins tells Eliza to turn out the lights. Eliza is becoming increasingly upset. Higgins can't find his slippers and Eliza picks them up and throws them at him. She says, "I've won your bet for you, haven't I? That's enough for you. I don't matter, I suppose." Higgins is angry and says that he won his own bet.
Eliza has finally had enough of being treated like an experiment and stands up to Higgins. Despite his academic intelligence, Higgins lacks the emotional or social intelligence to consider Eliza's own feelings. He doesn't see her hard work in having won the bet, only his own.
Eliza calls Higgins a "selfish brute," and says that now she will be thrown back "in the gutter," where she came from. Higgins refers to her as "the creature," and Eliza lunges at him with her nails. He stops her, calling her a cat, and throws her down into a chair. Eliza says she knows Higgins doesn't care about her at all. Higgins says that no one has ever treated her badly at his house, and says that Eliza must simply be tired after a long day.
Higgins continues to treat Eliza poorly, because she comes from "the gutter." While at times he has no patience for the Victorian social hierarchy, Higgins is still prejudiced against the lower class. As he calls her a creature and a cat, his misogyny is also a factor in his rudeness toward her.
Eliza regains her composure, but is still upset. She wonders what will happen to her now. Higgins tells her she will be alright, and suggests she marries someone. He offers for his mother to find her someone. Eliza thinks of this as prostitution and says she was above this even in her lower-class life.
The only way for Eliza to get out of her predicament, according to Higgins, is to marry someone wealthy. Yet Eliza is resistant to the traditional female roles of Victorian society, seeing this kind of marriage motivated by money as prostitution.
Higgins is annoyed by Eliza's comment, and tells her she doesn't have to marry. He says Pickering can set her up in a florist's shop. He starts to leave to go to bed, and Eliza asks him whether her clothes belong to her or Pickering now. She doesn't want to be accused of stealing anything.
Eliza's question about her clothing symbolizes her uncertainty regarding her identity: is she actually a new person now, or are the clothes merely a temporary costume covering her same lower-class identity?
Higgins is offended at the question, but Eliza says that she has to be mindful of such things, because she is a commoner. She says to Higgins, "There can't be any feelings between the like of you and the like of me." Higgins calls her ungrateful and tells her she has wounded him "to the heart." He leaves angrily and Eliza looks satisfied at having upset him.
Despite her apparent transformation, Eliza now says that she is fundamentally different from Higgins because of their different social classes. The unemotional, academic Higgins finally appears here to have feelings, as well.