The amount of details you can extract of the chroma key backgrounds is incrredible at 4k resolution.
Jokes apart, I second the idea that 4k makes sense for professional videographers either for ultra high def or post production to FHD. However, for consumers, I still doubt that theere is more than bragging rights attachsed tu 4k.
What if you already own a 4K TV, why wouldn't you want to be able to film home stuff in 4K?
Does Sony sell a 4K TV? Yes
Similarly for Panasonic, they make a 4K TV so they're also selling cameras capable of producing 4K.
I understand that companies will try to push products ecosystems, but from a practical standpoint, I don't see the benefit of a 4k video of your 5yo's birthday party or of your aunt gardening. I just feel like you need some pretty controlled condition to allow for 4k to produce significant additional details at the shutter speeds allowed in video. If you add the time and computer resources required to edit those videos, it makes even less sense to me.
As a professional tool, 4k surely has a place. As a consumer product, may be not as sensible a product. Nevertheless, consumers are not often acclaimed for their ability to make sensible decisions... there should be plenty of buyers.
However, i'll be honest and confess that is just my opinion and I'm not a video guru.