In Harper Lee's To Kill a Mockingbird, Jem changes in two ways in Chapter Four. First of all, he starts to become more protective of Scout. She gets home a half hour before he does. One day, running home, Scout notices something in the hole of a tree: it is a stick of gum, which she promptly puts in her mouth. When Jem arrives home and finds what she has done, he makes her spit the gum out, and gargle, threatening to tell if she doesn't comply—which she does.
Jem is also becoming more concerned about how he looks in the eyes of his peers. When Dill comes to Maycomb for the summer, the talk of "Hot Steams" arises (which is a old superstition about a fog that will kill you if you walk through it). Scout makes fun of the idea which embarrasses Jem. Soon, he takes his revenge by hurling the tire in which Scout is curled up (a game they play) as hard as possible.
Until it happened, I did not realize that Jem was offended by my contradicting him on Hot Steams, and that he was patiently awaiting an opportunity to reward me. He did, by pushing th tire down the sidewalk with all the force in his body.
However, even though Jem is angry with Scout, he still demonstrates concern for her safety when the tire rolls into the Radley yard. He screams and hollers for her to get out. When she does, he is mad that she did not bring the tire so she tells him to get it himself. He is fearful, but doesn't want to look like a coward, so he does.
In order to keep up the facade of his fearlessness of the Radleys, Jem suggests that they play a new game called "The Radleys" where they reenact the lives of the Radley family based on snippets of gossip and their imaginations.
Jem is becoming more mature in his protection of Scout, but he is also more concerned about how he appears to his peers, wanting to seem grown-up and fearless.
Jem was now in the fifth grade, and he had previously told Scout that she was "to stick with the first grade and he would stick with the fifth." The two children were growing up and apart. Scout was released from school 30 minutes before Scout, so their time together was diminished; he also seemed to be bossier. But he was mostly tired of playing their old Rover Boys game, and Scout saw that they had already grown bored on the first day of summer vacation. But Dill's suggestion that they begin a new game that centered around the Radleys triggered a new interest in Jem--not only in the game, but also in actually getting a look at Boo himself. Jem was getting bolder: One example was that his fear of the Radley house was leaving him, and lying to Atticus was another.