Converting .bnk videos to .ogg format.
Converting MPEG-4 Videos directly to Ogg Vorbis Audio Files (.mp4 to .ogg conversion) should be possible and relates to users attempting to extract the audio track (OGG) from an MP4 video.
It appears that your options are limited and that your best bet could be using the Freemake Converter, which allows users to import MP4 videos and re-save them in OGG format.