T4 has same dmg as khitai one, so bb sin do same dmg as t4 sin, T4 sin has more dexterity which gives sin more evade / stamina, it's not that game breaking,
BUT t4 dagger from etnity is just a joke.. the base dps is so damn high with that combat rating counted on it, not to mention the sin first feat from lotus tree makes it pretty silly..