Consider the following premises. u -> p_4. s <-> u. r <-> p_4. ~r. Can we infer ~s from them?
 From r <-> p_4, ~r we can infer ~p_4. From u -> p_4, ~p_4 we can infer ~u. Finally, from s <-> u, ~u we can infer ~s. Therefore, the answer is yes.