I saw Up, also an animated feature film, in 3D, too and it was roughly the same with the rather obvious layering of elements.
That there isn't anything physically captured could provide better opportunities to apply the technology.
Eventually, it just isn't that much and a nice gimmick when applied correctly.

4K in cinemas and in combination with 3D can mean that you get 2K per eye, a bit over 1080p each. It doesn't look bad though, even if 1080p isn't really that high definition anymore when you're used to seeing it about every day for a few years.
4K screens or UltraHD, which seems to be four times the resolution of 1080p, if I'm not mistaken, will probably take an eternity to be available on something similar to Blu-Ray Discs and I wouldn't count on a reliable service to stream crisp streams in the format with not much compression either, connection speeds generally not being that futuristic and companies wanting to be cost-effective.
Some content provider might offer something which won't look like LEGO deluxe and charge a lung, a testicle and an eye for it, but unless there isn't anyone actively profiting from decreasing quality in the streams offered to keep the data volume transferred as low as possible, it'll be some exotic premium model not delivering the full potential in terms of visual fidelity just like 1080p streams and most dowloaded video content in comparison with Blu-Ray Disc material today.
It's basic greed.

The Avatar screening I saw was in IMAX 3D and it was pretty good.
Since the projection was shot, edited and projected digitally, it didn't offer any details 70mm film might surpass 35mm film in and the sharpness looked the same compared to digitally projected features not in IMAX 3D to me. That's hardly surprising when the projector was the same a few times, I guess, so the actual difference, if there is one, wasn't obvious.