Joseph Heller's Catch-22 contains many important scenes and sequences that add meaning to the novel and help define the characters and their experiences. You are being asked to choose and analyze four particularly significant scenes. Let's look at some possibilities to help you get started.
You might select the scene between Yossarian and Doc Daneeka in which Yossarian learns about the Catch-22. Yossarian wants to claim insanity in order to get out of flying missions but actually claiming insanity means that Yossarian is not insane at all. You will have to explain how this scene introduces one of the novel's main themes and why it is so important for Yossarian (for it changes how he behaves throughout).
Another important scene you might choose centers around Yossarian telling Nately's prostitute fiancee about Nately's death. The woman's emotion is violent and shocking, and she attacks Yossarian. It is like she is trying to avenge the death of not only Nately but everyone else who has died in what seems to be a senseless war. Her senseless acts of violence stand as a metaphor for the war.
Third, you might focus on Chaplain Tappman's struggle with his faith and the questions he has about the war and his role in it. Pay close attention to chapters 25 and 26 for this one. This scene explores the role of religion in the chaos of war.
Finally, you will want to focus on Yossarian's choice at the end of the novel. He is told that he must choose between a court-martial and an honorable discharge, but the latter comes with another catch. He must officially approve and support his superior officers' policies. Yossarian says no to both and makes a third choice, finally overcoming the Catch-22.