GU4 Bi-Pin Base light bulbs are bulbs that have a two pin (Bi-Pin) base.
Bi-Pin Base bulbs are denoted by the letter G (as it applies to a base
determination, NOT shape which denotes a globe lamp or bulb). The letter
G is followed by a number which designates the gap width (from center
to center) between the pins of the base in millimeters. A G4 Bi-Pin
Base bulb has a gap width between its two pins (center to center) of 4 millimeters.