If we are to liken the idea of "enjoying themselves" with being happy, then Act 4 represents the scene in which most of the characters are happiest. In Act 3, the height of the climax takes place, which is the moment of greatest turmoil for the characters. While Puck is having fun and being greatly amused by his mistake of mixing up the lovers, the lovers themselves are very distraught. Even though Helena is now loved by two men instead of none, she is not enjoying the moment because she cannot bring herself to believe that the men are being sincere. Instead, she thinks both men are mocking her. Not only that, she believes her best friend since childhood is in on the joke, as we see in her lines:
Lo, she[Helena] is one of this confederacy!
Now I perceive they have conjoin'd all three
To fashion this false sport, in spite of me. (III.ii.195-197)
Helena's new distrust of Hermia of course causes a huge quarrel between the two best friends. Not only that, the two men are ready to fight over Helena, showing us that Act 3 is definitely the moment of greatest turmoil and distress, rather than of pleasure or happiness.
In contrast, Act 4 is the act in which the resolution begins. Oberon decides it is time to release Titania from the spell that is making her be in love with Bottom as a donkey. Also, since the spell has already served its purpose of distracting Titania so that Oberon can obtain the Indian boy he is jealous of, when Titania awakes, she and Oberon are now reconciled and are a friendly married couple once more. Also, Puck successfully corrects his mistake and pairs the four Athenian lovers correctly. Not only that, Theseus overrules Egeus's petition to kill his daughter and decrees that Hermia shall marry Lysander and Demetrius shall marry Helena. Hence, all four lovers are very content in this scene, and Theseus is pleased by the outcome. Thus, all major issues are resolved in Act 4, showing us that this is the act in which the characters are happiest and enjoying themselves the most.