For me, the correct lenght of mp4 videos is shown correctly as long as the movie has not more than 2GB...
I already opened a bug report ...
Unfortunately that will not be fixed anytime soon.
MC depends on 3rd party library support for this (TagLib) and the maker of that lib have added 2G+ support in there new v2 branch, but v2 is a total rewrite and is still unstable.
And since v2 have lots of API changes, It will require some work in MC too when going over to v2. So first a stable version of it needs to be released and then I need to find the time to rewrite how MC is using it.