What grim news is imparted at the beginning of Act 4 of Romeo and Juliet?

The news is divulged to Friar Lawrence by the county Parris at the friar's cell and concerns the fact that Juliet's father has decided that she should marry him that Thursday. Parris tells the friar that he could not convince Lord Capulet to delay the marriage and says:

Immoderately she weeps for Tybalt's death,
And therefore have I little talk'd of love;
For Venus smiles not in a house of tears.
Now, sir, her father counts it dangerous
That she doth give her sorrow so much sway,
And in his wisdom hastes our marriage,
To stop the inundation of her tears;
Which, too much minded by herself alone,
May be put from her by society:
Now do you know the reason of this haste.

Juliet is apparently so overcome with grief at Tybalt's death that she weeps copiously and is persistently sorrowful. Her father fears that she may do herself harm in this state as if of the opinion that a speedy marriage would cure her of this unfortunate ailment. The irony is that Juliet is weeping for her love, Romeo's, banishment and not so much for Tybalt's death. Furthermore, she is already wedded to Romeo and her father's insistence makes matters worse, not better.

When Juliet arrives, she indulges in a brief conversation with Parris in which she makes it quite clear that she does not love him and feels forced into marriage. Parris feels offended by her remarks but Juliet stands her ground, stating that what she said was done directly and honestly. At the end of their conversation the friar asks that Parris leave so that he may have a private moment with Juliet.

During their discussion, Juliet expresses her determination to avoid marrying Parris at all costs. She would even consider suicide. She asks the friar for a solution. She is prepared to do anything. The friar then suggests her taking a potion which would induce a death-like sleep. She will then be buried in her family's crypt and within forty two hours arise from her sleep when he and Romeo will be present to welcome her.

It is this desperate measure which leads to the most unfortunate and tragic events which are to unfold later, resulting in the demise of our hapless protagonists.