The major purpose of Act IV of this play is to act as the falling action in the plot of the play.
The climax of the play has come in Act III when Romeo kills Tybalt. This is the point where the conflict comes to a peak. After that, the conflict will begin to resolve and that is what happens in Act IV.
In Act IV, we see the consequences of the conflict start to unfold. We see Juliet reacting to the exile of Romeo. We see the stage set for the resolution of the play in Act V. This is done by the introduction of the potion that will put Juliet to sleep and allow her to avoid marrying Paris.
In this way, Act IV is the falling action. It is the playing out of the logical consequences of the conflict and it is the set up for the final tragedy of the play.