Consider the following premises. s_3. s_4 or ~s_3. s_4 -> s_3. Can we infer s_4 from them?
 From s_4 or ~s_3 we can infer s_3 -> s_4. From s_4 -> s_3, s_3 -> s_4 we can infer s_4 <-> s_3. From s_4 <-> s_3 we can infer s_3 -> s_4. Finally, from s_3 -> s_4, s_3 we can infer s_4. Therefore, the answer is yes.