What is the main theme of Act 4?

The main theme in Act IV is that of madness. Hamlet's madness feigned or real encompasses the entire play. None-the-less, in this act, we also witness Ophelia's descent into madness. Thus far, Ophelia has been represented as a chaste, innocent, obedient, bewildered little girl. With her madness, however, she suddenly has a deluge of lines and a rich, multi-layered, startling consciousness. The songs she sings are quite sexual – especially the one that begins, “To-morrow is Saint Valentine’s day.” This ballad, which documents the duplicity of a man who promises to marry a young maid in order to get her into bed, and then abandons her because she relented to him, has been read by some as evidence that Ophelia herself gave up her virginity to Hamlet, who then left her in the lurch. In Kenneth Branagh’s adaptation of Hamlet, for instance, the filmmaker explicitly shows flashbacks to Hamlet and Ophelia in bed.