What is the difference between GU-4 and G-4 bulbs?
The pin size is a little bigger in the GU-4 than the G-4. There's probably a risk that this could damage the socket, but they are pretty close. More importantly, I don't see anything that specifies the wattage for these bulbs, so it's possible that you replaced a lower wattage bulb in a higher wattage fixture.