The 4mm is TTI’s mid profile extension for the Staccato/Infinity/Springfield Prodigy/Atlas/Infinity 9/38 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.