What aspects of Juliet's character do you think are most apparent in act four?
A large part of this answer is going to be contingent on how the reader views Juliet. Based on the perception of Juliet and the development of her traits thus far in the work, the presentation of her in Act IV will be dependent on the reader. I think that this prefaces all responses. In the final analysis, how you have seen her thus far will have a large effect on how you see her in this particular moment. Some might point to her overwhelming despair when she weeps at a point "past hope, past care, past help." Others might point to her eternal optimism when she accepts the Friar's plan. Finally, her doubts before taking the potion could represent the complex setting whose difficulty has been exponentially increased with the feelings shared between she and Romeo.
Juliet is an honest person. She is honest to herself in that she refuses to give in to a marriage to Parris. She is honest to Paris in that she does not lie to him about her feelings for him.
"To answer that, I should confess to you." she says to Paris.
She is also desperate and suicidal over the idea of a union to Paris.
"Be not so long to speak; I long to die,"
She is willing to do whatever it takes to find relief from her situation. She loves Romeo and is willing to do whatever it takes to be with him. She is also hopeful and looks to the Friar for help. She longs for the time when she can be wed to Romeo.
Juliet is courageous in that she is willing to do whatever the Friar has in mind to be with her love.