Chapter 4 in To Kill a Mockingbird is the chapter Scout first finds something in the knothole of the tree by the Radley house. Coming home from school one day, she notices something shiny in the knot hole. It is two pieces of gum, a great treasure for a young child. She gobbles up both pieces, and when she tells Jem what she found, he makes her spit out the gum fearing that she may have been poisoned by Boo Radley. Later, Scout, Jem, and Dill start taking turns rolling down the street inside a tire. When it is Scout's turn, Jem gives her a big push, and she rolls into the Radley yard up to the porch. Scout is a little dazed, and Jem runs in to save her. At the end of the chapter, we learn that Scout hears someone laughing from behind the window of the Radley house.
The gum and tire incident are significant because, for the first time, the children realize that they have made contact with Boo. Chapter 4 sets up the next adventures they have in their attempts to see Boo. They start playing "Boo Radley", a game where they act out dramatic scenes of what they think is going on in the Radley house. Jem and Dill try to send Boo a note asking him out for ice cream through a window with a fishing pole. And finally, Jem and Dill sneak up on the porch but are chased off by a shadowy figure walking across the porch. Jem and Dill run and hear a shotgun blast go off over their heads, and in an attempt to get away, Jem catches his pants on the fence and has to take them off to get free.
For the reader, this chapter signals that perhaps the stories about Boo aren't correct and just made up folklore. It is our first look at Boo as a human being, and it starts to unravel the gossip about Boo as being insane and dangerous. This chapter foreshadows Scout and Jem's future encounters with Boo that include finding more things in the knothole, Boo covering Scout with a blanket during the fire at Miss Maudie's, and eventually, Boo saving their lives.