change metadata tag before importing mp4 into iTunes to Music Video
If I recall correctly, music videos have the following metadata:


genre: Music Video
media kind Music Video
kind Purchased MPEG-4 video file


As a minimum, I imagine you'd have to change the genre and media kind metadata so that they're labelled as Music Video to ensure the .mp4 files are imported as music videos. 

As an aside, you can typically view the above metadata within iTunes by doing a Get Info. That is, select the item and press command I (or you can go to iTunes > Edit > Get Info). This brings up all the metadata and you can view the:


genre within the Details tab
media kind within the Options tab
kind within the File tab