What are we fulfilling with the reading the megillah?
Recounting the miracle is a form of publicizing the miracle and thus takes on an amplified status among mitzvot d’rabbanan. The obligation is based on the word נזכרים – remembered. Thus reading the megillah is closely associated with another זכירה – remembering Amalek, and is a form of the mitzvah of זכירת עמלק.
The Megillah serves 2 purposes – it reminds us of the good that Hashem did for us and the bad that Haman tried to bring upon us.
Why do we read 4 verses out loud?
The mekorot that discuss making noise when Haman’s name is mentioned.