Consider the following premises. u_4 <-> s. s or ~u_4. Can we infer s <-> u_4 from them?
 From u_4 <-> s we can infer s -> u_4. From s or ~u_4 we can infer u_4 -> s. Finally, from s -> u_4, u_4 -> s we can infer s <-> u_4. Therefore, the answer is yes.