How to convert MKV files to MP4 files that are compatible with Whatsapp on Manjaro Linux and Le Chat

How to convert MKV files to MP4 files that are compatible with Whatsapp on Manjaro Linux and Le Chat

I have one input.mp4 file that is a MKV file.
Dropping it in a Whatsapp group will give error that file is not supported.
Using Ffmpeg tool and one line command, file can be converted.

Insstall on Arch linux:
sudo pacman -Syu
sudo pacman -S ffmpeg