@Entity
public class EntityOne {

    @OneToMany
    private Set<EntityTwo> entityTwoCollection;

}

@Entity
public class EntityTwo {

    @OneToMany
    private Set<EntityThree> entityThreeCollection;

}

@Entity
public class EntityThree {

    @OneToMany
    private Set<EntityFour> EntityFourCollection;

}

...
// 4, 5 ... 20