How did you import? Referenced or managed? Have you tried to repair your Aperture Library?
Sometimes repairing will retrieve lost media inside the library, if there are problems with lost references (illegal filenames, or something). Try to repair permissions and database:
however 4 videos (MVI_ format) have gone missing
Do the four missing videos have the same format, as the other videos that you imported that day?
All of the videos that I took and the 4 that have gone missing are the same format/type, from the same camera. I imported from the SD card through Aperture, as managed (to be a part of the 'Aperture library file). I've run 'Repair Database.'
Perhaps, they did not import correctly. I never double checked that the imported videos played, I just saw the thumbnail. Perhaps the import created a thumbnail, but did not import the video from the card?