How do we summarize Act 4 of Shakespeare's A Midsummer Night's Dream ?
Since none of the acts found in A Midsummer Night's Dream contain a fourth scene, I am going to assume that you actually meant to ask about Act 4, rather than Scene 4. Below is an answer concerning Act 4.
When summarizing anything, it is important to pick out the major points. Also, when summarizing fiction, it can be helpful to know about the elements of plot structure. For example, Act 4 is where the resolution of the play begins to take place. The resolution is the moment in a plot when the action begins to wind down and the author or playwright ties up any loose ends. In Act 4, one of the first things that is resolved is that Oberon decides it is time to release Titania from her humiliating spell of being in love with a donkey. Oberon tells Puck that when Oberon saw Titania in the woods gathering gifts for Bottom, Oberon insulted her until she cried, and after "she in mild terms begg'd [Oberon's] patience," he asked her for the foundling, which she gave him right away (IV.i.47-59). Since Oberon now has the child in his care, he decides the potion has served its purpose and enchants Titania with an anecdote. Hence, one resolution is that Oberon and Titania's quarrel is ended, Oberon gets the child, and Titania is released from her spell.
Another resolution we see take place in Act 4 is that Oberon instructs Puck to remove the donkey's head from Bottom and to make Bottom believe that what he had experienced was a dream so that he can return to Athens with the others fully repaired. Puck does indeed do as instructed. However, Bottom awakes half believing that what he had experienced was real and half believing it was a dream. Finally, Bottom decides that it was such an amazing dream that Quince should write a ballad about it that Bottom can sing at the end of their play. However, when Bottom meets up with his fellow actors in the second scene of Act 4, it appears he has changed his mind about telling anyone what happened to him for all he says when they ask is, "Not a word of me" (IV.ii.30).
A final resolution we see in this act is that when the four lovers are found in the woods, Theseus decrees that Hermia will not be put to death for refusing to marry Demetrius. Instead, Theseus decrees that Lysander will marry Hermia and that Demetrius will marry Helena, whom Demetrius was engaged to before he started pursuing Hermia. We witness Theseus overruling Egeus's plea in the lines, "Fair lovers, you are fortunately met; ... Egeus, I will overbear your will" (IV.i.178-180).