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.
