Why did Scout want to quit their game?
In Chapter Four of To Kill a Mockingbird, Jem decides that the group should play a new game: acting out the life of the reclusive Boo Radley. In this little drama, Scout plans Mrs. Radley and is responsible for sweeping the porch, while Dill plays Mr. Radley and must walk up and down the sidewalk. Jem plays Boo himself and hides under the porch steps, howling and shrieking. The kids rehearse this over and over, adding a plot and dialogue. Atticus eventually catches them in the middle of their game and responds to their secretive behavior by telling them that he certainly hopes the game has nothing to do with the Radley Family.
Thus, Scout cites two reasons for wanting to quit the game. First, on the day that she had gone rolling down the sidewalk in a tire and landed squarely in the Radley's front yard, she was certain that she had heard someone laughing from inside the house through all the chaos. Second, Atticus' arrival home and his discovery of their game makes her realize that they might be doing something wrong; Scout knows that Atticus would not be pleased if he discovered that they had lied to him about the theme of the game. This, however, is not a convincing argument for Jem, who decides that they can still play the game as long as they change the characters' names.
In Chapter 4, Scout wants to quit their game because she is sure that Atticus knows, and because she heard laughter from inside the Radley house.