Why is Paris at Friar Lawrence's cell at the beginning of Act IV?
In the play "Romeo and Juliet" by William Shakespeare, the Friar (Friar Laurence) is often blamed by audiences and critics for his "meddling" and interference. His outlook,however, on Romeo and Juliet's quandary is different from ours. Presumably, being a man of cloth, or a man of God, his driving motive in life is the promotion of good and the prevention of sin, So when he sees Paris looming in on his cell, he sees sin coming - trouble with a capital T. Because, you see, for Juliet to go ahead and marry Paris would now be bigamy and a dreadful sin at the time because of what the Friar has already done in marrying her to Romeo. He must act quicky and think fast if he is to avert mortal sin, as Paris has come to him to arrange the wedding.
Juliet trips on Paris on her way to the friar's. She has the intention of being there because she learned that she is now being forced to marry Paris.
It is ironic that Paris is there making plans for their wedding when she happens to be going there to figure out how to prevent their marriage. During the previous scene, she had mentioned she wanted to go to the friar for a remedy to her problem of having to marry him. Now, the problem stares her in the face as Paris discusses the when and where of the wedding and even tries to kiss Juliet.
Paris is there at Friar Lawrence's cell to talk about wedding plans. He is, as you know, supposed to be getting married to Juliet.
Juliet's parents have consented to the match and so what Juliet wants does not matter. Remember, Juliet's parents do now know that she is married to Romeo so they see nothing wrong with having her marry Paris. In fact, they think marrying will help her because they think she is mourning for Tybalt.
Friar Lawrence is now in a major spot -- he knows Juliet is married, but he can't tell.