At midnight on Wimpole Street, Eliza enters looking pale and tired, almost tragic. Pickering and Higgins ignore Eliza, talking about where Higgins's slippers are and whether there is any mail. They have been to a garden party, a dinner party, and the opera, and Eliza was extremely successful, fooling everyone. Higgins expresses his contempt for society and says that he is glad that the experiment is over, since he was beginning to grow tired of it. Pickering says that it is almost scary how good at it all Eliza is-she is better than the society ladies. Eliza, who has gone to find Higgins's slippers, begins to look angry, then murderous. Higgins leaves, asking Eliza to turn off the light and to ask Mrs. Pearce to make coffee in the morning.
Higgins returns, looking for his slippers again, and Eliza throws them at him. Eliza angrily explains that she does not know what to do with herself, now that she has won the bet. Higgins says that she is overreacting. He tells her that after she sleeps she will feel better. He adds that she is quite attractive, so maybe she could marry after all-perhaps his mother could find someone genteel for her to marry. Eliza responds that she was above selling herself when she was a working-class woman; she merely sold flowers instead of her body. Higgins replies that her moral judgment against marriage is unfair.
Eliza asks whether her clothes belong to her or Pickering, since he is the one who bought them. Higgins replies that of course they belong to her. When she protests that she did not want to be accused of stealing them, he is hurt. (She has not forgotten her roots in poverty.) He says that that her comment shows a want of feeling. Eliza pushes her advantage, asking him to take the hired jewels to his room so that they will be safe. Higgins exclaims that he would shove them down her throat if only he would not have to return them to the jeweler. Eliza also gives Higgins back a ring that he bought her, a piece of jewerly that was not borrowed. He angrily throws it into the fireplace and says that she has "wounded him to the heart."
Eliza is glad to get "a little of her own back." Higgins tries to regain his dignity, saying that he has lost his temper for the first time in a long time. He leaves the room in a controlled manner, but he slams the door on the way out. Eliza smiles, imitates his accent in a wild manner, and gets down on her knees to look in the ashes for the ring.
In this pivotal act, the relationship between Eliza and Higgins finally explodes. It is revealed that there has been a deeper feeling between them, and the fact that he has given her a ring certainly suggests a promise of marriage. This act also expresses Shaw's deepest condemnation of society, which is fleshed out more fully in Mrs. Warren's profession; that is, he puts in Eliza's words the idea that societal marriage is nothing better than the exchange of sex for money like what one sees among prostitutes. Eliza, if not also Shaw, views the upper-class marriage market as more degraded than her previous profession of selling flowers. From a class perspective, at least, her opinion expresses Shaw's deep socialism, supporting the claim that the working classes can and often do have more dignity than the hypocritical segments of the upper class.
Act Four also reveals an interesting power dynamic between Eliza and Higgins. Eliza most greatly resents the fact that Higgins views her success as his own, and she is infuriated by his idea that (like the mythological Pygmalion) he is the agent who created her. She views this claim as presumptuous and dehumanizing. Although by questioning Higgins about the jewelry she reminds him of the gap in class between them, she succeeds in making him angry. The ability to affect someone who holds himself maddeningly superior to her heartens her-she is glad to get "some of her own back" in this way. The relationship between the two now includes Eliza's pleasure at being able to hurt Higgins.
Eliza's actions at the end of the act remind the audience of the very real dilemma facing Eliza: what is she to do-stay or go? She mimicks Higgins, pleased that she has effectively gotten him angry, but she then begins to search, almost compulsively, for the ring that she has just discarded. This juxtaposition demonstrates that she still has feelings for Higgins, being not yet ready to throw away the sentimental token that he gave her. Searching for the ring also suggests an economic prudence on Eliza's part; her future is very unclear.