There is a lot happening in Chapter 4 of To Kill a Mockingbird. This is the chapter that highlights the end of Scout's first year in school, the return of Dill for the summer, and the first couple of gifts found in the knothole in the Radley tree. The children also start acting out the Radleys' scissor-stabbing story and get caught by Atticus. Based on these events, some themes could be friendship or childhood imagination, but two that have specific passages associated with them would be the beauty of summer and the neighbor, Boo Radley.
First of all, summer is Scout's favorite season. Summer is when life seems to happen for Scout--when she can run free, socialize, and just have fun being a kid. The following passage couples beautiful imagery with the whimsical musings of a young girl:
"Summer was on the way; Jem and I awaited it with impatience. Summer was our best season: it was sleeping on the back porch in cots, or trying to sleep in the tree-house; summer was everything good to eat; it was a thousand colors in a parched landscape; but most of all, summer was Dill" (34).
The above passage presents themes such as Jem and Scout's sibling relationship, traditions during summertime, and the fact that summer is directly connected to Dill. In fact, a good title for a theme here could be "summer was Dill."
Another theme centers around Boo Radley, of course. Not only are the kids feeding their superstitious mentality by playing out the Radley family stories; they have received gum and two old Indian-head pennies from the knothole in the big Radley tree. Scout feels guilty for playing out the Radley stories over and over again, so she seems a bit relieved when Atticus catches them. Scout explains why she does not want to play the stories again:
"Atticus's arrival was the second reason I wanted to quit the game. The first reason happened the day I rolled into the Radley front yard. Through all the head-shaking, quelling nausea and Jem-yelling, I had heard another sound, so low I could not have heard it from the sidewalk. Someone inside the house was laughing" (41).
This passage seems to be the beginning of a syllogism that helps Scout piece together the puzzle that is Boo Radley. First, if Mr. Nathan Radley goes to town each morning, and Scout heard someone laughing from inside the house, then Boo must be left alone in the house. Then, if Scout heard someone laugh that day, and it wasn't maniacal or angry, Boo must enjoy watching the children play. Finally, if there is a person in the Radley house who enjoys watching the children play, and gifts start showing up in the knothole of the Radley's tree, then the gifts must be from that person--none other than Boo Radley. A theme for all of this could be entitled "The Mystery Which Is Boo Radley" or something of the sort.
The kids are starting to cross a line from believing the neighborhood legend is the boogie man, to understanding that there is a human being in the house who may want to be friendly and kind to them. Another good theme associated with Boo Radley might be "nothing is as it seems," or "don't judge a house by its shut-in."