What does hamlet realize in act 4 scene 4?

Hamlet finally realizes that his duty to revenge is so great that the end must justify the means. He can no longer escape the necessity for action.

an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference.

it is the background information on the characters and setting explained at the beginning of the story. the exposition will often have information about events that happened before the story began. the exposition is often the very first part of the plot.