From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
G-4 is a type of Colonial plastic explosive, that usually uses an A-type detonator as an electric ignition source.
A-type detonators are often, but not always, used as a fuse for G-4, and possibly other explosives. They can be time-delayed or manually triggered.
Aboard ship they are typically kept in cases of 12 with slots labeled A-1 through A-12, which in turn are stored in small arms lockers (Water).