Note that avi, mkv and mp4 are all container formats.
avi and mkv can contain nearly any audio and video streams regardless of codec. mp4 can contain only a specific set of audio and video streams determined by a limited set of codecs.
You can trivially copy the audio and video streams directly over into an mp4 container provided that the codecs used are supported by mp4. As long as the codecs were supported also by the device you were looking to watch it on then you’d be able to watch it. So the codecs are the most important bit.
If I remember correctly, aac + h264 was commonly used for audio and video respectively, both of which are supported by mp4.
Of course it would still be an annoying extra step for someone who needed the file to be mp4, but at least being able to simply copy the streams is fast and cheap compared to streams that need to be transcoded. Transcoding could take painfully much time and could also easily further degrade the quality of the streams.
An important thing to note about mp4 is that in some configurations, metadata that is needed for playback is placed at the end of the file. This means you can’t playback this sort of file directly until you had downloaded the whole file completely. Often there would be a separate sample file that you could download to have a look at the quality anyway but still, it is one reason that you might want to go with avi or mkv instead as I don’t think those containers suffer from that problem at all. So if you download mkv and preview it early in let’s say VLC and you know from the nfo that they are using mp4-compatible codecs then you can both always have preview to ensure quality is good (or even that it is at all the movie you are looking for), and then after you’ve downloaded it you can copy the streams over in an mp4 container quickly and then consume the movie on your mp4 media device.
avi and mkv can contain nearly any audio and video streams regardless of codec. mp4 can contain only a specific set of audio and video streams determined by a limited set of codecs.
You can trivially copy the audio and video streams directly over into an mp4 container provided that the codecs used are supported by mp4. As long as the codecs were supported also by the device you were looking to watch it on then you’d be able to watch it. So the codecs are the most important bit.
If I remember correctly, aac + h264 was commonly used for audio and video respectively, both of which are supported by mp4.
Of course it would still be an annoying extra step for someone who needed the file to be mp4, but at least being able to simply copy the streams is fast and cheap compared to streams that need to be transcoded. Transcoding could take painfully much time and could also easily further degrade the quality of the streams.
An important thing to note about mp4 is that in some configurations, metadata that is needed for playback is placed at the end of the file. This means you can’t playback this sort of file directly until you had downloaded the whole file completely. Often there would be a separate sample file that you could download to have a look at the quality anyway but still, it is one reason that you might want to go with avi or mkv instead as I don’t think those containers suffer from that problem at all. So if you download mkv and preview it early in let’s say VLC and you know from the nfo that they are using mp4-compatible codecs then you can both always have preview to ensure quality is good (or even that it is at all the movie you are looking for), and then after you’ve downloaded it you can copy the streams over in an mp4 container quickly and then consume the movie on your mp4 media device.