Maybe this shows my ignorance, but mp4 is a video format not an image format. AFAIK the most common animated image format is gif, although you now get animated png
So obviously you could convert mp4 to animated gif or png - there are online converters for this, but if what you want is fro xenforo to do the conversion, then I don't think it is possible (yet).
But it's a bit confusing, the title of the thread says banners, so if that is waht is needed rather than gifs in posts, you could have an mp4 in a banner if you use an html video player - this could be in an ad position or in a notice. But again, it's a video not an animated image. I think video would usually take a lot more bandwidth than a gif