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