G4 halogen capsule bulbs are low voltage voltage bulbs. e.g. 6v, 12v, 24v, 28v. G4 bulbs run at a low wattage, with the maximum being 20w. The pins of a G4 bulb are 4mm apart.
The 12 volt G4 bulbs are very common in household lighting, often used in decorative light fittings which have multiple bulbs. They are also used in some small utility lights such as under cabinet lighting.
The other voltages are less common and are used in a variety of applications from torches, to bicycle lamps to commercial vehicles and boats.