Consider the following premises. s -> w_4. u_3 -> w_4. w_4 -> u_3. ~u_3. Can we infer ~s from them?
 From u_3 -> w_4, w_4 -> u_3 we can infer u_3 <-> w_4. From u_3 <-> w_4, ~u_3 we can infer ~w_4. Finally, from s -> w_4, ~w_4 we can infer ~s. Therefore, the answer is yes.