Intrepid is a tier four I thought.
T1: Miranda, Centaur, ShiKahr
T2: Nova, Constitution, Saber (counting their interchangeable variants)
T3: Akira, Cheyanne, Olympic (and variants)
T4: Defiant, Galaxy, Intrepid (ditto)
T5: Prometheus, Sovereign, Luna, and other not so pretty classes (double ditto)