The CompXtension 4″ allows you to shorten your pcc barrel to 12.5″ while also retaining a fully functional compensator.  The shorter barrel reduces “dwell time’ thus curtailing the force acting upon the bolt.  In a blow back pcc, this lowers bolt speed, resulting in a softer shooting rifle.  Because 9mm velocities tend to drop in between 10-12″, both the 4″ & 6” versions allow very efficient use of pressure while also retaining the gas necessary to make effective use of the compensator ports.