User blog comment:TheDeadDragon/The Bow?/@comment-4212769-20120501065735

The 3rd charge is mostly unlocked. For the 4th charge you need an armor with Capacity Up. In MHFU for example the Yama Tsukami gunner armor, in MHP3rd the Jhen Mohran (low rank only), Amatsumagatsuchi and Deviljho armor.