mp4 only sound no video

okay so i have a problem. i have had this before but i honestly forgot how to fix it 😭 okay so i downloaded an youtube video in 4k to make an edit, as an mp4. it used to work but suddenlt i only have sound and no video when i import it which was weird cuz just before that i made an edit out of a mp4 file. so i import the mp4 file that did work before and suddenly it doesnt show video either? but then i look further and i somehow got an m4v version of it which did work. so now im trying to find out how to download youtube videos in 4k as an m4v or

mov file without it being like a billion gbs. its so odd because i have no idea how i did that before and i think it was on accident then but its nowhere in my search history either so.. does anyone know what to do?