Thanks, so the question then is how do I ensure my video is in the format they require???Desmond, mp4 is just a container format and doesn't say anything about how the content is encoded. It can contain video, audio and subtitle tracks. The video can be encoded using e.g. MPEG-4. Usually the container format dictates what encodings can be used for both video and audio.
And I guess the output from iMovie is a mp4 container using h.264 encoded video as this is what is supported by e.g. AppleTV.