Shooting video: why does it time out?
Maybe the memory card buffer fills up even if you did not reach the 4GB limit. I have a 7D and a x133 CF card, and this card can't handle the video data transfer rate from the camera, so the buffer fills up faster than card writing speed most of the time.
The indication that what you had was a card buffer problem is the 4-level bar that appears on the screen while shooting video. This bar only appears if the buffer is soon to be filled.