A G4 light bulb is a bi-pin base lamp that runs off a low-voltage power supply. “G4” is a base designation which means the base has two pins and the gap width between the pins (center to center) is 4 millimeters. The G4 light bulb finds its usage in a variety of applications where space is a premium.
A G9 light bulb is a line voltage operated capsule bulb having two spades or wire loop contacts spaced 9 mm apart. The G9 base was exclusively reserved for bi-pin halogen bulbs that operate at high voltages (120-130 volts in the US, and 220-240 volts in EU and China).