Act IV takes place in the chambers of Honoria Fraser in Chancery Lane, where Vivie has returned to work after leaving the cottage in the country. Frank is already there, and soon Vivie enters. She hasn't been expecting him and gives him a cold welcome. He invites her out, but she refuses because she has a lot of work to do before bed.
Frank sits down and asks Vivie what she has been doing. She explains that Honoria rehired her gladly before leaving for a long vacation. She is grateful to be back at work and claims that she will never take another vacation. Frank does not approve of this decision.
He brings up the revelation that Crofts sprung on them before Vivie left: that they are half-siblings. He says it cannot be true, since he does not feel for her at all the way he feels about his other sisters. The Reverend Samuel Gardner has also denied that it is true.
Vivie says it makes no difference to her, since she thinks of him as a brother anyway. This hurts Frank's feelings, but he resolves quickly to never flirt with her again out of respect for her. He assumes she must have a new lover, but she denies it.
Praed arrives to say goodbye before he leaves for Italy. He tries to engage Vivie in a discussion of the country's beauty, but she is repulsed by the idea of all the artwork and culture he is going to experience. She becomes nearly hysterical in telling them to stop trying to induce her to be sentimental, given what she now knows about her mother's profession.
Praed scolds her for being judgmental, but in doing so reveals that he is unaware of the extent of Mrs. Warren's current involvement in prostitution. She decides to reveal her mother's secret to the two men by writing it down on a piece of paper. They read it and are taken aback, but react calmly and reassure Vivie that their opinion of her has not changed.
Vivie leaves the room for a moment and Praed and Frank reflect on the information she has revealed to them. Frank tells Praed he is no longer interested in Vivie romantically, but not because of any moral issue; rather, he couldn't bear to inherit any of Mrs. Warren's ill-begotten money. Praed assumes this means he will never seen Vivie again, and Frank laughs at such an absurdly dramatic idea.
The theme of practicality versus sentimentality is at work in the conversation between Vivie, Frank, and Praed in this act. Praed is on his way to Italy, where he plans to indulge his artistic sensibilities. Vivie, in contrast, is triggered into an outburst by the idea of such a trip. Frank makes fun of her for being dramatic, and she accepts this, saying, "It's good for me. It keeps me from being sentimental" (93). Here, she reveals that although she is practical by nature, this personality trait is also something she nurtures within herself as a conscious choice.
Frank, too, reveals himself to be practical rather than sentimental in his reaction to the news of Mrs. Warren's true profession. He tells Praed that he cannot marry Vivie now, not for any moral reason, but because he does not wish to benefit from her mother's money. Ever dramatic, Praed assumes that this means he will never see Vivie again. Frank responds, "I can not understand the absurd consequences you romantic people expect from the most ordinary transactions" (96). Of course he will see Vivie again, since they have practically determined the bounds of their relationship. She sees him as a brother and he will return her affection unromantically.
Before telling the men her mother's secret, Vivie addresses them in a way that points to their male privilege. She tells them, "If we three are to remain friends, I must be treated as a woman of business, permanently single and permanently unromantic" (93). The fact that she even has to make this statement marks the difference between her experience as a woman and their experiences as men. They have the privilege of assuming others will treat them with respect; she must demand this, even of her two friends.
Her decision to remain unmarried is unconventional, as is her decision to share her mother's profession with Frank and Praed. Even as she makes the latter decision, she battles with her own conformity to convention: she cannot bring herself to speak the words that describe her mother's profession, so she writes them down instead. Before doing so, she complains, "There is nothing I despise more than the wicked convention that protects these things by forbidding a woman to mention them" (94).
After reading Mrs. Warren's secret, Praed demonstrates that he is able to practice the morals he preaches. Earlier in their conversation, he said to Vivie, "though I know that your mother is an unmarried woman, I do not respect her the less on that account. I respect her the more" (94). After Vivie leaves the room, when both men know Mrs. Warren's profession, he warns Frank that if he decides to leave Vivie on account of it, he will be acting "very despicably" (96).