This is all my opinion - I am no expert. I'm feeling to lazy to rewrite it with different phrasing, so I'm just adding a disclaimer.
I doubt we're going to see 4k tv's (there's already a bunch of claims that you can't tell the difference between 720p and 1080p unless you have a really, really big tv, and aren't sitting to far back from it.
But the big advantage is the ability to crop after you shoot the video. Say you film the scene with no zooming. Then later you want that "slow zoom" effect. Are you going to hire a bunch of actors to come back a refilm it? With higher resolution - you just crop in slowly via post-processing. And you don't lose any effective resolution because bluray and hdtv isn't going to see more than 1080p anyways.
Or sometimes something that seemed like a neat effect at the time doesn't work later - you thought slowly zooming in would be really great, unfortunately it turns out that the scene before it and the scene after it ended up doing the same thing, and the effect is now waaaaaay to much and distracting. With more pixels you can post-process your scene to not have any zoom, without losing resolution (because again you started with more resolution than you needed).
Or imagine that you shot it with everyone in the frame, but it's more intense with just two of the people...you get the idea.
