What is the most difficult thing to understand in Act 4 of Romeo and Juliet?

What is most difficult to understand in this Act is friar Laurence's position. He is faced with a moral dilemma, for he has already married the two lovers and now has to marry Juliet a second time, which cannot be, for it would mean that the he is complicit in committing a crime and an immoral act.

The fact that the friar even married the two lovers in the first place is problematic. He knew the circumstances in which the two found themselves and yet performed the marriage ritual, knowing full well that both families would have resented his actions and opposed it. They would have asked for the marriage to be annulled if that were at all possible. 

The friar is caught in a dilemma and has to not only ensure the two lovers' happiness, but he has to protect himself for he may be severely sanctioned for his actions. His solution would resolve the problem, but ignores the sorrow that will ensue. The friar is supposed to seek the goodness of all those he serves and not be the origin of their pain and sorrow.

Furthermore, even though he has good intentions, his current plan will scupper everything good he had originally planned. His idea was that a marriage between Romeo and Juliet would bring to an end the feud between the two families. His solution, however, may further widen the gap between the two and cause greater bitterness and resentment.

The expression: 'The road to hell is paved with good intentions' is most apt in this situation. The friar is naive to believe that his solution will work. As it is, his contrivances border on deceit and illegality, since he is assisting a criminal, for Romeo had committed a crime and had been banished. Added to that, he is helping the two in what may be deemed immoral. His function is to foster morality and, as a servant of God, he has a duty to act in the best interests of all concerned and not lead them down the path to perdition.

Furthermore, his suggestion that Juliet take a powerful sleeping-potion makes the vulnerable and obviously anxious teenager suspicious of his intent, as she states in scene 3:

What if it be a poison, which the friar
Subtly hath minister'd to have me dead,
Lest in this marriage he should be dishonour'd,
Because he married me before to Romeo?
I fear it is: and yet, methinks, it should not,
For he hath still been tried a holy man.

The only reason she really takes the potion is that she refuses to be married to Paris and would rather be dead. She, therefore, does not much care at this point for, to her, whether she dies or not, the outcome would be the same.

In this regard, then, it is difficult to understand why the friar does not seek an alternative solution. He obviously has some authority with both families and is respected. Why does he practice deceit and assist Juliet in presenting a lie? There obviously has to be a better way.

It is tragically unfortunate and deeply ironic that, in the end, the friar's wish does come true. The Capulets and Montagues end their conflict but at a great price: they lose their most precious gifts - a son and a daughter.