Energy Stone level 5 is the highest Energy stone. It can be use to fortify any level of equipments.
Energy stone lvl 2 is usually use to get +1
Energy stone lvl 3 is usually use to get +2 (or lower)
Energy stone lvl 4 is usually use to get +3 (or lower)
Energy stone lvl 5 is usually use to get all +4-+12 (or lower)
The higher the level of your equipments, the lower success rate it is and harder to get.
And remember to use Divine amulet when you're fortifying level 5+ equipments to higher level, as it prevent the equipments to drop a level.