One of the best and fastest ways to convert an MKV file to MP4, in our experience, is with the help of the Permute app. All you have to do is open the app and drag and drop your MKV. Next, at the top left of your app window check that the conversion format is set to MP4 (it’s likely going to be a default) and hit the little play icon at the bottom to start the conversion.
Your converted MP4 will be saved to the same location that your original file was dragged and dropped from. You can change this in the app’s settings to ‘always ask’ or select a new default folder.