Consider the following premises. p_19 <-> s. p_11 <-> p_4. p_19 -> p_4. p_11 -> s. Can we infer p_4 <-> p_11 from them?
 From p_19 <-> s we can infer s -> p_19. From p_11 -> s, s -> p_19 we can infer p_11 -> p_19. From p_11 -> p_19, p_19 -> p_4 we can infer p_11 -> p_4. From p_11 <-> p_4 we can infer p_4 -> p_11. Finally, from p_4 -> p_11, p_11 -> p_4 we can infer p_4 <-> p_11. Therefore, the answer is yes.