Consider the following premises. forall y_4: Q_2(y_4) or Q(y_4). exists y_4: T_3(y_4) <-> Q(y_4). forall y_4: Q_2(y_4) <-> T_3(y_4). Can we infer exists y_4: T_3(y_4) <-> Q(y_4) from them?
That is one of the premises. Therefore, the answer is yes.