The M4V Converter tool from DRmare looks promising as it's a comprehensive solution that can not only convert M4V videos to MP4, MKV, etc. but also unlock the 24-hour limit from the rented M4V movies.
iMazonKit Music Converter
- Download Amazon Music playlists Offline Losslessly
- Batch Convert Amazon Music Playlists to MP3, AAC, FLAC, etc. at 5X Speed
- Retain Original Audio Quality and Full Metadata of Amazon Music