The 4mm is the mid profile extension for the Staccato/Infinity/Springfield Prodigy/Atlas 2011 style magazine. Although it will work on all 2011 style magazines, it is recommended for the 140mm magazine. It brings the capacity up from 20 rounds to 22 rounds with the factory spring and follower and still fits the USPSA 140mm gauge. 
 
*Due to the variances in mag tube dimensions from different companies and even different generations of mags from the same company, the base pad may require minor fitting to the tube and the back of the lips may need the sharp edges buffed down so the mag will fit the gauge.*