Consider the following premises. t_4 -> s. t_4 <-> s. Can we infer s <-> t_4 from them? If possible, name the inference rules being used at each step.
 From t_4 <-> s we can infer s -> t_4 via biconditional elimination. Finally, from s -> t_4, t_4 -> s we can infer s <-> t_4 via biconditional introduction. Therefore, the answer is yes.