[maybewhenthesun]

That depends on the specific MP4 I think. My camera creates MP4s which are completely unwatchable without the ending. I found out the hard way when the battery died while recording my children's school play.

The only way I could save the file was to create a new movie with the exact same camera settings (luckily I hadn't changed anything on the camera) and graft the ending of the newly created mp4 onto the old one using some special utility a hero on the internet had created.

[necovek]

In general, some tools like mplayer or VLC allow playing an MP4 file without an "index", but may require certain CLI arguments.

There are tools which can fix that too (was it mconvert from mplayer or ffmpeg, not sure).

[maybewhenthesun]

This is not always true. If the 'index' is all that is missing the only thing is that you cannot seek. That's easily fixable. In the case of the files my camera writes a lot more info is only at the end. I can't remember what exactly but at least the resolution and the framerate were only stored at the end. And maybe even codec settings I can't remember.

I looked at it very thoroughly back then (it was a video I did not want to loose). Without grafted on info from a video with identical settings the video could only be played back as 8x8 squares of random noise.