Back at Olivia's house, Maria and Toby have locked Malvolio in a dark chamber to cure his "madness." Outside the chamber, Maria instructs Feste to put on a gown and beard she has procured and to pretend to be a "great scholar" (4.2.10), "Sir Topas." Maria goes to fetch Sir Toby while Feste puts on the gown. When they return, Sir Toby is delighted: "the knave," he says, "counterfeits well" (4.2.19).
The subplot of the prank on Malvolio is coming to a peak. Costume changes and role playing on stage also create another play-within-the-play, drawing attention to the theme of performance.
"Sir Topas" goes to the door of Malvolio's cell. Malvolio tries desperately to enlist him as an ally; Sir Topas parries his every attempt, telling him that the dark room he is in is really light as day. Malvolio begs Sir Topas to test whether or not he is mad by asking him a question. Sir Topas asks, "[W]hat is the opinion of Pythagoras concerning wild fowl?" Malvolio replies (correctly) that Pythagoras argued that the human soul might just as well inhabit the body of a bird. Sir Topas asks what Malvolio thinks of this; Malvolio (who, we will recall, is a Puritan) says that he thinks nobly of the soul and does not in any way approve of Pythagoras' opinion. Sir Topas replies that Malvolio must remain in the darkness then: he will not be let out until he believes that a human soul could just as well inhabit the body of a bird. With this "Sir Topas" leaves Malvolio, who cries out after him.
Sir Topas' question about Pythagoras is pseudo-intellectual, thus parodying Malvolio's pretentiousness. Yet Feste's Q&A with Malvolio also reflects the themes of inconstant love and changing identity. Over the course of the play, the powerful love that Orsino and Olivia claim to feel is directed at so many different people that it becomes meaningless—it might as well be aimed at a bird. Similarly, Sebastian and Viola/Cesario's identities are also confused multiple times, raising questions about the constancy of a "soul" or of one's internal nature. If a mere disguise can transform Viola into a man, why can't her soul inhabit a bird?
Sir Toby, amused, mocks Malvolio's desperate cries. But he confides in Maria that they must find a way out of this prank to avoid irritating Olivia any further. The two sneak off to Sir Toby's bedroom.
Here, in a rather unromantic off-stage culmination of the low plot, Toby and Maria get together.
Back in his clown personality, Feste returns to Malvolio's cell. Malvolio begs Feste to bring him a candle, pen, ink, and paper, so that he can write a letter asking Olivia for help. Feste agrees to deliver the letter, but first dallies for a while, teasing Malvolio.