Use VLC Media Player . It is free, open-source, and can play both MKV and MP4 files regardless of whether the extension is named correctly.
Does the video play correctly for you now, or are you getting a "format not supported" error? How to Convert MKV to MP4 01. Mom And Daughter.mkv.mp4
Because of the potential format confusion, it is best to use a "universal" media player that can handle almost any codec. Use VLC Media Player