Flashplayer 9 (Released June of 2006) and above support .mp4 video. If you are required to use a .FLV, simply change the extension. Flashplayer determines the video playback from the metadata in the file, not the file extension so it will play back the .mp4 properly.
If your end user still have Flash Player 8 installed on their machine, this will not work. You can test for player version with Flash or javascript and recommend an update if needed.
Zachary Guidry
Multimedia Developer