It may possibly help if you clear defaults. There may be a way to address just MP4 files, but this is what I would do.
First determine which app is loading.
From a home screen (not within an application) hit the menu then select "App Settings"
Move down to find the app and click on it.
Click on the "Clear defaults" button.
This will remove all defaults for the app, and so you may be prompted for other file types as well, but you can always select the app and select either Always or Once.
If this doesn't work, you may not have another app loaded with the ability to process MP4 files.