You can use avidemux,just open the file and choose the container you want,video and audio options are on copy by default.
MKVtoolnix has a tool to extract the separate streams of a MKV in case you only want the video or something.
Would be a good idea to use mediainfo first to see if the video stream is actually something compatible with mp4,most are x or h .264 but MKVs could have anything in them.