If I'm not mistaken, as a container format, MP4 videos should be able to contain videos that are 480p. Also, from what I am able to find, the MPEG-4 video format does not have a limitation that prevents 480p either.
Yet, MP4 videos on YouTube are only available at 360p, 720p, and 1080p, and 480p is only available as FLV or WebM (regardless of the video).
Am I missing something? At 4:3, a 480p video would need to be 640x480, but at 16:9, it would be 853.333…x480. Could that be the problem? Is there a tangible limitation preventing 480p MP4s?
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To technically answer your question: Yes. MP4s can be 480p.
There's nothing that prevents a certain picture size from being created with MP4 containers. I don't know the upper boundaries, but you could theoretically have a 1 × 10000 video too, for example if it was computer generated. The x264 encoder has more realistic boundaries – a video won't look good if its dimensions are not divisible by 8, and the height must be divisible by 2.
As for YouTube, this article suggests that earlier on, fmt code 18 was used for 480p MP4. The code is still available, but now it's used for 360p MP4 instead, which suggests a change in the backend implementation of YouTube's content management.
Note that when watching over a browser, you still get the FLV version, which means that this version must be available in all resolutions.
WebM is not always available, but in most cases for all resolutions. Google does this to promote their WebM project, obviously but there's another reason.
MP4 content is only served when the client uses YouTube's HTML5 video, but only for Apple's Safari or IE9. All other browsers actually support WebM. Therefore, encoding content in MP4 has a very low priority.
If you think about the fact that one hour of video are uploaded every second to YouTube, this does make sense from an economical standpoint.
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