Consider the following premises. ~p_2 or u_3. p_2 or q_3. q_3 -> q_4. q_4 or ~u_3. Can we infer ~q_4 from them?
 From q_4 or ~u_3 we can infer u_3 -> q_4. From p_2 or q_3, ~p_2 or u_3 we can infer q_3 or u_3. Finally, from q_3 -> q_4, u_3 -> q_4, q_3 or u_3 we can infer q_4, which contradicts ~q_4. Therefore, the answer is no.