Curley's wife has several purposes in Chapter 4. First, as throughout the book, her sexuality shows a different kind of power than the money, muscle, and skills that the men respect.
Second, she disrupts the rising dream of the men. In particular, she shatters Crooks's attempt to control his destiny and his dignity; she shows how the hidden structures of race can always rise up.
Third, on the level of plot, her continual return shows that she will create great trouble...which she does, soon.