Consider the following premises. q_4 -> t. r_3 -> q_4. t -> r_3. r_3 -> t. Can we infer q_4 <-> r_3 from them?
 From r_3 -> t, t -> r_3 we can infer r_3 <-> t. From r_3 <-> t we can infer t -> r_3. From q_4 -> t, t -> r_3 we can infer q_4 -> r_3. Finally, from q_4 -> r_3, r_3 -> q_4 we can infer q_4 <-> r_3. Therefore, the answer is yes.