Converting MKV to MP4 is one of the least painful things in the format mess, because half the time you do not even have to re-encode.
- Remuxing vs re-encoding: remuxing just repackages the video, seconds and lossless; re-encoding recompresses everything, slower with a small quality loss. - HandBrake: the classic, free, three clicks, handles batch folders. - Shutter Encoder: FFmpeg power with buttons on top, the nerds' pick for friends. - VLC: yes, it converts too, which always surprises people. - FFmpeg: one terminal line, no progress bar needed. - CloudConvert: the browser option when you cannot install anything.
Skip uploading a 4GB file to an online converter just to change the container. An hour of uploading to save three minutes of clicking is a trade nobody should make. And if what you really need is short clips from a long recording, ReelCut turns your video into captioned vertical shorts for $4.99 a video.