The theme that dominates this chapter is alienation. All the four characters that appear are suffering from alienation, and all as a result of a particular "disability" in the eyes of society.
Crooks disability is his color, and he is segragated from the rest of the workers because of it. He is unhappy and often hostile, having always lived in hostile environments. As a result, Crooks acts out against others, alienating them in turn. This is why he taunts Lennie in Chapter 4, even though Lennie doesn't judge him.
Lennie is also alienated, because of his mental disability. While Crooks is removed physically from the rest of the staff, Lennie is removed intellectually. He doesn't know how to communicate with the others, besides George, and is often left behind in conversation.
Candy's disability is physical, revolving both around his age and his hand. Like the dog he wants so much to save, Candy has reached a point in his life where he can not contribute materially to the work that the other men are doing. He is so protective of his dog because he fears a similar treatment. This is why Candy grasps on so tightly to the dream of the farm - it is a place where he would be both wanted and useful.
Finally, there is Curley's wife. As a woman, she is also alienated in this world dominated by men. She has no friends and few rights, and like Crooks, she acts out with hostility when threatened.