Chapter 4 is the climax of John Steinbeck's novella Of Mice and Men. In this chapter, the brutality of Lennie's strength is finally revealed as he kills both the puppy he had longed for and Curley's wife, who was looking for someone to pay attention to her.
The first major event in this chapter occurs right at the beginning. In the great barn, Lennie is sitting with a "little dead puppy that lay in front of him." Lennie had killed the puppy from "bouncing" it too hard. This scene with the puppy makes it evident that Lennie will never care for the rabbits he cries about.
The second major event that occurs in the chapter is Curley's wife being able to explain why she is so unhappy on that ranch. She explains that she had so many opportunities to make it out of where she's from, but instead married Curley. She talks about how she "Coulda been in the movies, an' had nice clothes—all them nice clothes like they wear."
The third major event to occur is Curley's wife asking Lennie if he wants to stroke her soft hair. Lennie gets too rough and when she tells him to stop, he panics and snaps her neck. While Lennie's potential to be dangerous is foreshadowed throughout the book (he kills a mouse in Chapter 1, smashes Curley's hand in Chapter 2, and kills the dog at the beginning of this chapter), this scene makes it clear that Lennie is a danger because he does not understand the consequences of his actions. After killing Curley's wife, Lennie is only worried about one thing: George. He says over and over a variation of this phrase: "George'll be mad."
The fourth and final major event in this chapter is when George and Candy see Curley's wife dead in the barn before telling the rest of the guys. The most telling part in this scene is Curley's seeming lack of care for his dead wife. Instead of sobbing or showing any type of sorrow, Curley's first reaction is "I'm gonna get him. I'm going for my shotgun. I'll kill the big son-of-a-bitch myself."
All of these major events lead to the novella's necessary resolution, the mercy killing of Lennie, which happens in the next chapter.