[FFmpeg-user] "Padding" start and/or end of video

[Richard Branham]

I have four mp4 recordings of a video conference feed.  After the
conference ends, the files are multiplexed into a single video with each of
the original files displayed in a quadrant of the output window.  This
works as expected if all four videos are of the same duration.

If the primary user starts recording at time T0 and the other 3 join at
X seconds later, I need the individual videos in the muxed file to start at
T0 + X.  I also need the primary user's video to continue even after a
subsequent user leaves the conference and their video ends early.

My first thought is to lengthen the shorter videos by concatenating a black
screen to the beginning and/or end to make the shorter video the same
length as the primary one and to offset the short video to the correct
point in the conference recording.

Is this the best way to go, or is there a more straightforward approach?

Thanks!
Richard