ffmpeg -i out.mp4 -i 1.mp4 -i 2.mp4 -i 3.mp4 -i 4.mp4 -filter_complex " nullsrc=size=640x480 [base];[0:v] setpts=PTS-STARTPTS, scale=320x240 [upperleft];[1:v] setpts=PTS-STARTPTS, scale=320x240 [upperright];[2:v] setpts=PTS-STARTPTS, scale=320x240 [lowerleft];[3:v] setpts=PTS-STARTPTS, scale=320x240 [lowerright];[base][upperleft] overlay=shortest=1 [tmp1];[tmp1][upperright] overlay=shortest=1:x=320 [tmp2];[tmp2][lowerleft] overlay=shortest=1:y=240 [tmp3];[tmp3][lowerright] overlay=shortest=1:x=320:y=240" -c:v libx264 output.mkv

Cette video aura une taille file de 640*480 Chacune des vidéos aura comme taille 320*240