In Act IV, how will the Friar relay the news of his plan for Juliet to Romeo? When she returns to her father, where does Juliet say she's been,...

In the beginning of Act IV Juliet is desperate to avoid going through with the arranged marriage to Count Paris. Her father and even the Nurse have given her no options, so she turns to Friar Laurence, showing up at his cell in Act IV, Scene 1. The Friar comes up with a plan for Juliet to fake her death. Once she is presumably dead, she will be placed in Capulet's tomb where Romeo will rescue her once she awakens and take her away to Mantua.

The Friar plans to send a message to Romeo informing him of all of this. Later, in Act V, the audience learns that Friar Laurence sent Friar John to deliver the message but Romeo never received it because John was delayed by a plague threat in Verona. 

Juliet agrees to the Friar's plan. When she returns to her father in Act IV, Scene 2, she apologizes to Lord Capulet, tells him she has been advised by the Friar to beg forgiveness, and that she will go through with the marriage. She says,

Where I have learned me to repent the sin
Of disobedient opposition
To you and your behests, and am enjoined
By holy Lawrence to fall prostrate here [Kneeling.]
To beg your pardon. Pardon, I beseech you.
Henceforward I am ever ruled by you.

Capulet is overjoyed by his daughter's change of heart and he is so excited at the prospect of having her married to Count Paris he moves the wedding from Thursday to Wednesday, which is the next day. When Lady Capulet questions whether there is enough time for preparations, Capulet says he will stay up and make sure everything is ready. He says,

I’ll not to bed tonight. Let me alone.
I’ll play the housewife for this once.—What ho!—
They are all forth. Well, I will walk myself
To County Paris, to prepare up him
Against tomorrow. My heart is wondrous light
Since this same wayward girl is so reclaimed.

Of course, one of the reasons that the day is moved up is a plot vehicle for Shakespeare to further complicate the Friar's plan. It turns out there is simply not enough time for the Friar to reach Romeo with a message about his plan, especially after the first message was delayed.