The G4 and GU4
both have the same diameter pins and pin spacing. The difference is that the
GU4 typically involves a pair of grooves on the sides of the lamp for a clip to
hold the lamp in place. Think of a JC10 Halogen Capsule and an MR11. They both
have the same pin diameter and spacing, but the MR11 has a groove on each side
for retention (since it’s a directional light) and needs to be aligned more so
than a halogen capsule.
The original MR11 leds needed more driver space so Halco had to go with the G4. The
new one is more advanced and they were able to squeeze everything into a smaller
space and make more light.
