Okay, so I have some mp4 video clips with a separate alpha channel. There is one file that is the actual video, as in what I want to see, and then the other is is black with white for the parts I do not wish to be transparent. I am not sure how to combine these in Final Cut so as to be able to put them on top of another video. I have already tried doing it under the modify menu, both with the clips (the video and alpha channel) directly on top of each other, and with them is separate lines. I have not been able to get it to work at all. Any help will be appreciated. Thanks!
What you have in the B&W file is a matte.
You need to put your image on layer 3, the matte on layer 2 and the background you want on layer 1.
Then set your picture (layer 3) as "Travel matte LUMA".
But start by converting your files to Prores.
FCP doesn't likes MP4.