From True Ames: 

There is a reason this is the most iconic, functional and versatile single fin available...it works on just about anything! The Greenough 4-A has been our bread and butter for close to 40 years now, and is still the most popular single fin design there is.

A truly well-balanced fin...the full base coupled with the tapering, moderately flexible tip provides a solid amount of drive off the bottom, and an acute responsiveness through arcing turns. Designed for both speed generation and spending time surfing in the pocket, the 4-A is a fin that will hold when you need it to and release when the time is right. A fin that can do it all, throw one in your board and feel the success!