The mood in Act 4 can generally be characterized by danger, desperation, and fear.
We first see Shakespeare present the mood as dark and foretelling of upcoming danger in Paris's speech explaining why Lord Capulet made the decision to have Juliet marry Paris so suddenly. Paris explains that Juliet is grieving so severely that Lord Capulet thinks "it dangerous / That she do give her sorrow so much sway" (IV.i.9-10). The term "dangerous" points to the dangerous and foreboding mood in this scene.
We see Shakespeare present the mood as also being a desperate mood later in this same scene when Friar Laurence responds to Juliet's threats of suicide. Friar Laurence tells Juliet,
I do spy a kind of hope,
Which craves as desperate an execution
As that is desperate which we would prevent. (IV.i.69-71)
The term "desperate" shows just how desperate Juliet is to escape her marriage to Paris, and how equally desperate Friar Laurence is to prevent both of them from committing a grievous sin through a second marriage.
Finally, the mood can also be characterized by fear. Juliet is very afraid of her future. More specifically, Juliet is terrified of faking her death and dying for real, or even waking up alone in the tomb surrounded by all of her dead kin. We see Shakespeare present fear as a mood in Juliet's lines, "I have a faint cold fear thrills through my veins / That almost freezes up the heat of life" (IV.iii.16-17). The word fear in these lines certainly serves to characterize the mood in this scene as a fearful one because the scene is approaching pending doom.
Thank you soooo much! I now have a much better understanding of this act.