If you prefer not to install software, you can use web-based tools, though these require uploading and downloading the file:
For a fast "remux" (changing the container without losing quality), you can use this command if you have FFmpeg installed: ffmpeg -i 00004.mkv -codec copy 00004.mp4
Services like Maestra AI , HappyScribe , and Vizard.ai allow you to upload an MKV or MP4 and receive an editable transcript or .srt subtitle file.
If your goal was actually to extract the (speech) from the video file into a document:
CloudConvert or Zamzar are popular options for standard video conversion.