ffmpeg how to map 4.0 sound FL FR FC and B

And trying to encode Edwards Scissorhands that had 4 channel sound, I think FL FR FC and B, Trying to put the result in a opus output. I’m failing so far with:

ffmpeg -i Input.mpg -filter_complex "[0:a]pan=4c|c0=FL|c1=FR|c2=FC|c3=BL[a]" -map "[a]" -ac 4 -mapping_family 2  -b:a 196k -c:v libsvtav1 -preset 4 -crf 35 -c:a libopus -vf crop=704:528:8:24 -map 0:v:0 Output.mkv

Any ideas?