Consider the following premises. p_4 -> t. t -> q. p_6 -> p_4. q -> p_6. Can we infer ~(p_6 <-> q) from them?
 From p_6 -> p_4, p_4 -> t we can infer p_6 -> t. From p_6 -> t, t -> q we can infer p_6 -> q. Finally, from p_6 -> q, q -> p_6 we can infer p_6 <-> q, which contradicts ~(p_6 <-> q). Therefore, the answer is no.