How convert mkv to mp4

•  convert an input video file into a MKV file using VP8 encoding with VORBIS audio encoding

           ffmpeg -i input_video.avi -f matroska -vcodec vp8 -acodec libvorbis output_video.mkv

• convert a video into a video using VP9 for video encoding and OPUS as audio encoding

           ffmpeg -i input_video.avi -f matroska -vcodec vp8 -acodec aac output_video.mkv