Not all video editing programs support mkv files, but OBS Studio (Open Broadcaster Software) has a built-in way to convert (or, more accurately, “remux”) mkv files to mp4 files. Here’s how to do it:
Open OBS, click File, then Remux Recordings

This video was created using OBS Studio 24.0.1

The Star Citizen mkv footage seen in this video was used in my Carrack "commercial" which can be found here: https://www.youtube.com/watch?v=1tI0wJDIQe8