ffmpeg -re \
	-vf \
	-i dummy_background_video \
"
movie=video1.mp4 [m1];
movie=video2.mp4 [m2];
movie=video3.mp4 [m3];
movie=video4.mp4 [m4];
[in][m1] overlay=0:0 [x1];
[x1][m2] overlay=W/2:0 [x2];
[x2][m3] overlay=0:H/2 [x3];
[x3][m4] overlay=W/2:H/2 [out]
"