suggestion: Use split to avoid repeating the input

[varenc]

In your example, instead of repeating the same input 4 times, you can make it more concise by just using the split filter to make multiple copies of the same input:

ffmpeg -f lavfi -i testsrc -filter_complex \
"[0:v]split=4[v2][v3][v4],negate[a]; \
 [v2]hflip[b]; \
 [v3]edgedetect[c]; \
 [v4][a]hstack=inputs=2[top]; \
 [b][c]hstack=inputs=2[bottom]; \
 [top][bottom]vstack=inputs=2[out]" -map "[out]" -c:v ffv1 -t 5 multiple_input_grid.avi

Cool tool! Cheers