combine 4 clips into one 2 x 2 grid video mosaic

quad.sh

#!/usr/bin/env bash
finalwidth=$(ffprobe -v error -select_streams v:0 -show_entries stream=width -of csv=p=0 $1)
finalheight=$(ffprobe -v error -select_streams v:0 -show_entries stream=height -of csv=p=0 $1)
halfwidth=$(echo "$finalwidth/2" | bc)
halfheight=$(echo "$finalheight/2" | bc)
x="x"
ffmpeg -i $1 -i $2 -i $3 -i $4 -filter_complex "nullsrc=size=$finalwidth$x$finalheight [base]; [0:v] setpts=PTS-STARTPTS, scale=$halfwidth$x$halfheight [upperleft]; [1:v] setpts=PTS-STARTPTS, scale=$halfwidth$x$halfheight [upperright]; [2:v] setpts=PTS-STARTPTS, scale=$halfwidth$x$halfheight [lowerleft]; [3:v] setpts=PTS-STARTPTS, scale=$halfwidth$x$halfheight [lowerright]; [base][upperleft] overlay=shortest=1 [tmp1]; [tmp1][upperright] overlay=shortest=1:x=$halfwidth [tmp2]; [tmp2][lowerleft] overlay=shortest=1:y=$halfheight [tmp3]; [tmp3][lowerright] overlay=shortest=1:x=$halfwidth:y=$halfheight" -c:v libx264 quad_vid.mp4

• dependencies: must have ffmpeg/ffprobe installed for your OS, must have bash shell (OS X or Linux)
• this script expects each video file to be the same dimensions, and makes the final video duration the length of the shortest input video, and final video dimensions the same as the input videos, final rendered video is mp4 format
• usage: download this file quad.sh, and place in a directory containing your video files. To make executable open a terminal window, type cd <yourdirectory>, then chmod +x quad.sh Then, to run, type ./quad.sh <video1> <video2> <video3> <video4>

Here's an online version of the tool
https://sonoclipshare.com/quad_video/