Consider the following premises. ~u. p_8 <-> s_4. p_8 <-> u. Can we infer ~s_4 from them?
 From p_8 <-> u we can infer p_8 -> u. From p_8 -> u, ~u we can infer ~p_8. Finally, from p_8 <-> s_4, ~p_8 we can infer ~s_4. Therefore, the answer is yes.