The ride is apparently projected at 10k, but could be upscaled to that (most likely). They dance around actual render resolution in that article, and I really doubt it's a coincidence. The angle of view in FoP isn't all that much larger than a modern theater's main auditorium. Definitely not double, width wise, if you sit in the sweet spot for movies. Add in that you are moving, which is going to severely hinder your ability to discern pixels, and I still highly doubt 95% of viewers would notice if it was 4k. And the other 5% probably sit 15 feet from their awesome 4k TV at home.
I'm just a loon right? Well, let's dig into details.
Using this distance graph, for 4k, the slope is about 8 feet per 120 inches.
https://www.thx.com/assets/uploads/2017/01/1-ghF7Unj-vk60wyKUiVy4EQ.jpeg
FoP's screen is said to be 70' x 100' so the diagonal is about 1500", assuming no curve. So for 4k to be fully seen at 1500", you'd need to be at least within 96 feet of the screen. For a 1080p image (half resolution of 4k), it looks about half the slope. I don't know for sure, but we'll assume 8k is half the slope of 4k. If we extrapolated that, then for an 8k resolution, at 100", you would need to be about 4 feet, or 60 feet for FoP. And I'll be darn, the center of the screen is about 30 feet from the center seats, of that. But, the farther you get out, you quickly start getting 60 feet away for parts of the screen. So, it does seem most seats are close enough to see 8k, with some assumptions.
However, I felt the edges, that some people are VERY close to, are blurry, and anyone not in the center seats is more than 60 feet from the other side of the screen.
Add in the screen is curved, and actually MUCH wider, by linear inches, than 100 feet, so it's sorta unclear what the horizontal resolution is. 10k should be the horizontal resolution, but that would assume 100 linear feet of screen width, when it is probably closer to 150
So, taking in to account the motion, the fact most folks are likely more than the maximum distance away to truly even see 8k in half of the screen (especially taking into account 2/3 of the people are above or below the sweet spot by a good distance), and I have a real hard time believing they rendered at 8k. It would have been quite a waste.
I once found this INCREDIBLE video about what you eye can really see. By the cinematographer of The Last Jedi, among other films. It's very long, but is about as visual fact based as I've seen. Ultimately, it proves that even in a local theater 4k is unnecessary. And those screens can get to be equal to 1/2 the dimensions of the FoP screen. A GREAT watch, if you're a big time theater nerd.
http://yedlin.net/ResDemo/ResDemoPt2.html
What does all this mean? They may be projecting in 10k (not sure how, since I can't see it being more than 2 4k projectors on top of each other. Hell, it may have even been rendered in 10k (but I just don't believe that, maybe 5k per eye). But, ultimately, even 4k would have been enough for the vast majority of people, especially at 60 fps, and taking in to account your eyes are moving MUCH more than they do in a regular theater. Plus, they are looking through old 3D glasses with scratches, and multiple layers of distorting plastic in them.