Quote Originally Posted by alton View Post
Bad ballast or sometimes depending on the wiring two lamps are run in series with the ballast, so if a wire comes loose both lamps quit working. Sounds like you are going to have to send it back.
I would think more likely the wiring issue, if they are all on the same wire series that would mean they are all on the same ballast. I doubt a ballast would only work two of four bulbs if it went bad. Most likely you'd loose that whole chain of bulbs for bad ballast.