FFMPEG Stitch four videos together, then add overlay to result
Add the overlay to the result of the stacked videos.

Use

ffmpeg -i input0 -i input1 -i input2 -i input3 -i overlay -filter_complex \
"[0:v][1:v]hstack[top]; \
 [2:v][3:v]hstack[bottom]; \
 [top][bottom]vstack[v];[v][4:v]overlay=0:0[v]" \
-map "[v]" output.mp4