Act 4 serves as falling action - events that led from our conflict (turning point) to the resolution. In this scene, all of our relationships are exactly where they should be for the first time in the play. Puck has corrected his Lysander/Demetrius mistake, so Lys and Her will be married as will Dem and Hel. In addition, now that Oberon has acquired the Indian boy, Titania has had her spell removed and is in love with Oberon instead of Bottom again.
So, dramatic purpose as it relates to falling action? Our four lovers are rightfully paired, no more confusion, and will be married (resolution) in Act 5. Oberon and Titania are no longer fighting. Theseus has solved the original problem posed by Egeus in the first act. Last, but certainly not least, our over-actor, Bottom, has decided to add an epilogue to their performance at the wedding.