Folder Action to convert with ffmpeg a whole subfolder of videos
I actually came to a solution. The script

1) recieves a folder

2) gets folder contents repeating for subfolders

3) filters for mp4

4) runs a shell script:

for f in "$@"
do
    DIRPATH=`dirname "$f"`
    FILENAMEEXT=`basename "$f"`
    FILENAME="${FILENAMEEXT%.*}"
    EXTENSION=${f##*.}
    cd "$DIRPATH"
    mkdir -p WMV MP4
    /volumes/'macintosh hd'/applications/ffmpeg -i "$f" -b:v 4000k "$DIRPATH/WMV/$FILENAME.wmv"
    mv "$f" "$DIRPATH/MP4"
done

say "finished"