-filter_complex "[0:v]scale=1920:1080[tmp0]; [1:v]scale=1920:1080[tmp1]; [2:v]scale=1920:1080[tmp2]; [3:v]scale=1920:1080[tmp3]; [tmp0][tmp1][tmp2][tmp3]concat=n=4:v=1,setpts=PTS*0.5"