Consider the following premises. u or ~s_4. q or s_4. q -> u. s -> ~u. Can we infer s from them?
 From u or ~s_4 we can infer s_4 -> u. From q -> u, s_4 -> u, q or s_4 we can infer u. Finally, from s -> ~u, u we can infer ~s, which contradicts s. Therefore, the answer is no.