Consider the following premises. p_4 <-> p. p_6 -> t. t -> p. Can we infer p_6 -> p_4 from them?
 From p_6 -> t, t -> p we can infer p_6 -> p. From p_4 <-> p we can infer p -> p_4. Finally, from p_6 -> p, p -> p_4 we can infer p_6 -> p_4. Therefore, the answer is yes.