Consider the following premises. p_1 <-> q_6. ~p_1. r_4 -> q_6. q_6 -> r_4. Can we infer ~r_4 from them?
 From r_4 -> q_6, q_6 -> r_4 we can infer r_4 <-> q_6. From p_1 <-> q_6, ~p_1 we can infer ~q_6. Finally, from r_4 <-> q_6, ~q_6 we can infer ~r_4. Therefore, the answer is yes.