Sitting in Olivia's garden, Sebastian is enjoying the bliss of being loved by a beautiful and rich countess, although he is still thoroughly confused about why all this has happened to him. As he sits alone, he admires the lovely pearl which Olivia has given to him, and he wonders why Antonio did not meet him at The Elephant Inn, where they had agreed to meet. All of this seems truly like a dream; yet, looking at the pearl, he holds tangible proof that this is not a dream at all. He wishes that Antonio were with him to advise him; he heard that the sea captain did stay at the inn. Yet where is he now? And he wonders if the beautiful Olivia is mad — and, of course, there is another possibility: perhaps he himself is mad.
Olivia enters with a priest and tells Sebastian that she wants him to accompany her and the priest "into the chantry" (a private chapel). There, "before him / And underneath that consecrated roof," Sebastian will "plight [Olivia] the fullest assurance of [his] faith." Sebastian agrees to marry Olivia; the marriage will be kept secret until later, when they will have a splendid, public ceremony, befitting Olivia's rank. They exit, arm in arm, for the private ceremony, as the fourth act comes to a close.
The audience can readily sympathize with Sebastian's confusion and astonishment over the course of events that have taken place, and at the same time they can vicariously experience the great bliss of being loved. Sebastian tries to question reality, but he looks at the pearl that has been given him, and we must remember that Olivia is a person of great beauty; one could easily fall in love with her on first sight. For some modern critics, Sebastian's love for Olivia might strain one's belief, but we must remember that this is a romantic comedy, set in faraway Illyria, and Sebastian himself questions the plausibility of the events. The mistaken identities are, of course, a stock element of romantic comedies, and the forthcoming marriage between Olivia and Sebastian will provide the basis for all of the complications that will be unraveled in the next act.