Act IV represents a catalogue of different moods as the rising action of the play raises the tension and the audience is made to believe that firstly the forces opposing Romeo and Juliet's love are too powerful and then that they might have a chance of happiness. For example, in Act IV scene 1, it is clear that the mood is one of bleak, suicidal despair as Juliet is forced to face marriage with Paris when she is actually married to Romeo. She clearly states that death would be preferable to her than marriage:
I long to die
If what thou speak'st speak not of remedy.
This mood of course changes to relief and hope after the Friar shares his plan with Juliet and she begins to believe that there is a solution to the problems that she and Romeo face. Note how she responds to the Friar's plan as he offers his potion to her:
Give me, give me! O, tell not me of fear!
Juliet is too overjoyed at the potential solution to worry about fear in any way. This mood of elation of course is exchanged with despair and grief in Act IV scene 4 when Juliet apparently "dies," and so throughout Act IV the audience sees rapid interchange of emotions as the action accelerates towards its tragic close.