"Superior Two-Weapon Fighting [General] [Fighter]
You have reached the limits of the two-weapon fighting style.
Prerequisites: Dex 21, Greater Two-Weapon Fighting, Improved Two-Weapon Fighting, Two-Weapon Fighting, base attack bonus +16.
Benefit: You get a forth attack with your off-hand weapon, albeit at a -15 penalty.
Special: A ranger who chose the two-weapon fighting combat style can select this feat, even if he does not have a high enough Dexterity for it (though the other prerequisites must be met). But he can only use this feat when he is wearing light or no armor."
21 dex not 23 and 16 bab not 20