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