Consider the following premises. u_4 -> s_4. p_1 or u_4. p_1 -> t_4. s_4 <-> t_4. Can we infer s_4 from them?
 From s_4 <-> t_4 we can infer t_4 -> s_4. From p_1 -> t_4, t_4 -> s_4 we can infer p_1 -> s_4. Finally, from p_1 -> s_4, u_4 -> s_4, p_1 or u_4 we can infer s_4. Therefore, the answer is yes.