ffmpeg -i input1 -i input2 -i input3 -i input4 -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.mp4
