Okay girls and boys, HELP. I thought I understood

Okay girls and boys, HELP. I thought I understood what 4K would do and not do. Then one of my daughters asked me why I wanted to get a 4K TV. Understand, this child is a professional digital cinematographer who works mainly in HD. Her points are that if the viewer is watching a 4k film or 4K live event it will be all good. But, if you are streaming or watching cable non-4K you are likely to find that the picture does not fill the 4K screen or is grainy. I have no idea. If you have a 4K TV and have experience with non-4K content on your nice shiny TV, your comments and observations would be most appreciated.