Consider the following premises. forall y: R_4(y) or ~T(y). exists y: R_4(y) <-> T(y). Can we infer exists y: T(y) <-> R_4(y) from them?
 From exists y: R_4(y) <-> T(y) we can infer exists y: R_4(y) -> T(y). From forall y: R_4(y) or ~T(y) we can infer forall y: T(y) -> R_4(y). Finally, from forall y: T(y) -> R_4(y), exists y: R_4(y) -> T(y) we can infer exists y: T(y) <-> R_4(y). Therefore, the answer is yes.