Most modern video converters should have no trouble with converting MKV video to MP4 format, i.e. transformation of Matroska videos (.mkv files) to MPEG-4 format (.mp4 files). Any of the programs listed below can be used to convert mkv to mp4 or vice versa.
Open MKV file Open MP4 fileA video converting tool for macOS / OS X
An iPhone ringtone creator
A video editing software
A video conversion software
A video conversion software
A multimedia converter for formats suitable for iPad tablets
A Software for MP4/3GP file conversions
One of the many multimedia converter programs for Windows
Easy to use video converter for Mac
A fully-featured video converter for Mac
A fully-featured video conversion program
Universal video converting software for Mac
Universal All Purpose Free Media, Audio & Video Converter Player Recorder
Video editing software for Windows
A powerful and free video converter
A desktop transcoder app for Windows