This chapter serves as a point for all the most broken, forgotten and expendable characters to get together. Crooks, Candy and Lenny get together and portray the weakest and most dependent parts of the ranch. Crooks is a black ranch hand ostracized racially and handicapped with a broken back. Candy lost his hand to his years of hard labor and is marginalized due to his advanced age. Lenny is not right mentally and is completely dependent on those around him. Together they nearly hatch a scheme, but as noted before, nothing will change, as the opening and closing scenes show.
Chapter 4 begins and ends with Crooks putting liniment on his back in his room. He's isolated from the rest of the men and pretty much stays to himself in his room when he isn't working. First Lennie and then Candy goes into Crooks' room, and Crooks, for just a little while, forgets the racism that alienates him and talks to Lennie and Candy as men. When Curley's wife comes in, all of this ends. The men try to get her to leave, and she gets angry and plays the racism card at Crooks when he stands up to her. She puts Crooks back in his place as the only black man on the ranch who isn't allowed to even socialize with the whites. Her cruelty reminds Crooks of his situation and that he can't do anything about it. This is why Crooks tells Candy he really didn't want to go work with them on their farm.
This chapter begins and ends with the same event to show that nothing will change for Crooks, and it foreshadows that nothing will change for Candy, Lennie, and George.