The literary device that is most consistently used throughout act 4 of Romeo and Juliet is dramatic irony. Dramatic irony is when the reader/audience knows something that the characters do not. This device is used all the way through act 4.
In act 4, scene 1, Paris comments about how much Juliet has been weeping for Tybalt's death (4.1.6–15). While this is partially true, the audience knows that Juliet is also weeping for Romeo's banishment. During the entire exchange between Paris and Juliet, the audience knows that Paris's insistence on getting married will only push Juliet further away from him. Paris and Lord Capulet believe that Juliet's wedding to Paris will cheer her up and make her forget her grief for Tybalt. However, the audience knows that the prospect of a wedding only makes Juliet more upset and desperate.
In act 4, scene 2, Lord Capulet is relieved to find Juliet in a better mood and ready to marry Paris. The audience knows that Juliet has actually worked out a way to escape this marriage, but Capulet does not know this.
In act 4, scene 5, the Capulet family discovers that Juliet is "dead." The audience knows that Juliet has actually taken a sleeping potion that makes her have the appearance of being dead, but the Capulet family falls for the trick and has her buried.
Dramatic irony is used throughout act 4 and positions the reader to have additional knowledge of the thoughts and motivations of the main characters. It builds interest in suspense in act 4 and prepares the audience for the play's dramatic outcome.