I'm using xvid4psp video converter. But I can't convert the mkv file to an mp4. what do I do?
Are you able to play the MKV files?
Do you have Haali Media Splitter installed?
Without it Windows does not know how to deal with MKV files and therefore xvid4psp will not either.
MediaCoder converts from about anything to about anything. Codecs are all contained within so you don't need to install anything else.
I've had success converting various video formats including MKV to MP4 (M4V) using HandBrake.
Like others suggested, using another converter is one of the ways to solve the problem.
I would like to add (I havn't used the below converters ever but if you want to try it anyway below is the link)
I use and recommend "ImTOO Video Converter" but its not free
Freemake video converter will do your job easily and its support many formats and very easy to use.