Been trying a few hours now and can't hardcode and overlay a subtitle stream over the video stream. The video which is in a MKV format has two subtitle streams, #3 and #4. How do I go for the #4 stream instead of the #3 (default) one?

Here is my code
>ffmpeg -i "sora.mkv" -t 10 -filter_complex "subtitles=sora.mkv,[0:v]overlay[v],[v]scale=720x405[v]" -map "[v]" -map 0:a:1 -c:v libvpx -crf 10 -b:v 1M -c:a libvorbis -b:a 192k -minrate 192k -maxrate 192k sora.webm