Catherine meets with Isabella for the first time in three days. Isabella tells Catherine that she has received a letter from John, stating his intention to propose to Catherine. Catherine is quite shocked, and tells Isabella that she must write John and tell him the truth, that she does not love him—with her apologies if she misled him accidentally. Isabella briefly defends her brother, then agrees. Catherine points out that they will still be sisters, and Isabella makes the odd comment that "there are more ways than one of our being sisters." This is an allusion to the possibility that they might both marry a Tilney, since Isabella is now flirting with Frederick Tilney, and Catherine is falling in love with Henry Tilney. Catherine misses the remark and its meaning. Isabella says she is not angry with Catherine for refusing John, and that she could not guess Catherine's mind in the past. Isabella assumes that although Catherine flirted with John once, she has now changed her mind about him.
Frederick Tilney arrives and begins to flirt with Isabella, who flirts back. This bothers Catherine. She believes that Frederick is falling in love with Isabella, who is unconsciously encouraging him. Catherine does not think that Isabella is consciously encouraging him, because in her innocence she believes that Isabella would never cheat on James. Catherine gets tired of their flirting and, uneasy, leaves them alone.
Catherine keeps her eye on Isabella for several days and discovers that Isabella gives Tilney as much attention as she does James. James begins to suffer visibly, but Isabella seems to take no notice. Catherine becomes concerned for James, for Isabella, who is being scandalous by flirting with Frederick, and even for Frederick himself, because she thinks he is falling in love with a girl he can never have.
Catherine asks Henry to convince Frederick to leave Isabella alone, but Henry refuses, suggesting that Isabella is aware of what she is doing. He also suggests that her brother might be offended by her request, since James would probably like to think that he could keep Isabella's affection with or without Frederick's presence. Henry consoles Catherine, reminding her of her confidence in her brother and Isabella's love and assuring her that Frederick will soon leave Bath.
Again, Catherine's inability to read people causes her to miss important developments. When Isabella tells Catherine that she is not angry with her for refusing John, and that sometimes women flirt and then change their minds, she makes insinuations that Catherine misses. Catherine says that her opinion never changed because she never felt anything for John, but Isabella ignores this and says she would never think of rushing Catherine into an engagement merely to make her brother happy. Catherine does not pick up the parallel Isabella is drawing: just as Isabella would not want to pressure Catherine, the comparison goes, Catherine should not pressure Isabella to marry James. Isabella is also speaking for herself when she says that women often flirt and then change their minds. Isabella wants to make Catherine feel guilty for leading John on, so that when Isabella breaks off her engagement to James, Catherine will have no grounds for complaint. This plan fails, however, because Catherine is still too innocent to understand Isabella's insinuations.
In Chapter IV, Catherine's innocence leads to a mistaken understanding of the motivations of Isabella and Frederick. She even feels bad for Frederick because she assumes that the engagement between Isabella and James is unbreakable. Henry finds this innocence endearing, but he warns her that Isabella and Frederick will do as they please. Catherine comes to realize that perhaps Isabella is not truly attached to James. She tries to get Henry to tell her what he thinks, but he refuses, perhaps because he does not want to hurt Catherine by telling her the truth.