Of course, you can display watermarks in a random way, but for that you need to use Watermark Master. The program doesn't support MP4 format so far, but in a few days we will release its new version with such functionality.
If you want to use WM for displaying watermarks in a random way
If you use VideoCharge, it isn't possible to get effect of random appearance for watermark. You can only specify time of displaying and disappearing for it - to do that, use watermark editor, by selecting ... (see attached image)
In appeared window on Life Time tab specify time of displaying and disappearing for watermark. (see attached image)
Now we're working on version with an ability to display watermarks in a random way.