I have recorded some 4K videos as MP4 file on Panasonic VX878 and 9xx. Both produce a video which, when played in VLC or other player on Linux, has 3 tiles: 2 next to each other horizontally at the top 20 % of the canvas, both displaying the actual video, and the rest displays the 1st second of the video, looping.
I wonder where is the issue: In the player on in the camera? When I played the video in the camcoder's playback, it looked fine.
Yes, the videos are broken. They are created due to an overheat and some kind of built-in reduction of the recorded video to FullHD; but the resolution of the result is still 4K, and the last filled content of the 4K buffer is used to fill the bottom large part (hence the repeating).
Still, it's possible to get a FullHD video from the broken file:
FILTER='
[in] split [a][b];
[a] crop=1920:540:0:0 [ac];
[b] crop=1920:540:1920:0 [bc];
[ac][bc] vstack [stack];
[stack] il=luma_mode=interleave:chroma_mode=interleave'
if [ "$2" != "" ] ; then
ffplay -i "$INPUT" -vf "$FILTER"
else
set -x
ffmpeg -i "$INPUT" -filter:v "$FILTER" $QUALITY_GOOD_H265 -c:a copy "$NEW_NAME" \
&& touch "$NEW_NAME" -r "$INPUT"
set +x
fi