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.
TidiKit Music Converter
- Download Music, Playlists for Offline Playback
- Convert HiFi Songs to MP3, FLAC, WAV, etc. at 5X Speed
- Keep 100% Lossless Music Quality and ID3 Tags