I was thinking about how there will eventually be 4K movies available, and I wondered what that means for films like LoTR which, as far as I know, are only 2K. It seems that 4K will be more oriented for more recent, high-end films, and that many famous titles like Star Wars are as good as they can be on Blu-Ray.

Okay I'm not gonna claim to be entirely educated on this but will take a stab at it.  First of all LotR was shot on film, while the Hobbit trilogy is shot digitally, with the couple of links I've pulled up saying 5K.  (Here's a list of films shot digitally from Wikipedia, of which Star Wars Episode II is the first blockbuster release... in a financial sense anyway.)

I'm not sure at which point digital resolution theoretically surpasses traditional film in quality.

What I'm more concerned about is whether 4K will render current Blu-rays virtually obsolete, like Blu-ray did to DVD.

Even films shot on film could have problems if their effects and grading weren't done at high resolutions. A search does seem to suggest that the LotR films were indeed completed at 2k which would mean effects would have to be recreated, rendered or upscaled. It would be a lot of work. I'm surprised because I'd have been stunned if any movie is mastered in anything less than 4k. Although there was that Star Trek Motion Picture Director's Cut which was only made in SD, which was utterly baffling.

Can't help wonder if anyone would truly notice the difference between our current HD picture and 4K when it's in your sitting room...

Diminishing returns ^2.

With 4K you reach the level of current tech in cinemas (one that was only introduced when cinemas changed to digital projectors which was yesterday in cinema time, 2011), of course getting stuff making good use of that is going to be a rather big problem.

Ah good point (and sorry if that was your original meaning, Aran).  Now that you mention it, I seem to remember reading they completely redid the effects for Star Trek: The Next Generation for its Blu-ray release, for that very reason.

Still, for any movie produced in the digital age you'd think they will have held on to all their 3D assets, and it would simply (okay maybe not so simple) be a matter of re-rendering and re-compositing them.  Re-rendering should be pretty much automatic... I wonder how much manual effort goes into re-compositing.

Seems like it's a largely matter of whether people's TV sets get big enough.  Which given today's consumer culture might not be that unlikely.

Perhaps personal displays (Oculus Rift and the ilk) might eventually be able to put that kind of resolution to use?

My room mate and I have begun watching movies and playing games on a projector, so 4K resolution may be noticeable for us when use up a large section of a wall as our screen.