Wazzles said:

Correct me if I'm wrong, but if a 4K scan wasn't going to be used for a 4K release, instead being released in 1080p, wouldn't it not really matter what resolution the CGI is in? Wouldn't it look fine when scaled back down to 1080p?

Not only that, but pretty much all movies finished as a 4k DI only have their digital vfx rendered at 2k. That's why movies with a lot of vfx shots are only finished at 2k.

Digital effects shots are only rendered at higher resolutions for when they actually stand to gain from it. The literal handful of hollywood movies shot partially in native Imax had the effects for those scenes rendered at 5.7k or higher. I know that some of ILM's beauty shots of Pandora in Avatar were rendered at 3k (odd res, I know) so that they would really shine in the 70mm Imax blow-up.