Where did you come across the idea it is only being released in 2k? It was filmed on IMAX cameras, so why wouldn't it be released in IMAX format at IMAX theaters?
It didn't show up on the 4K list here:
Of course, realorfake4K doesn't list it as 2K either. There is an informational black hole when it comes to what resolution Infinity War was mastered at. But usually when information isn't forthcoming that means it's not 4K.
Same with theaters. The ones with 4K projectors will openly advertise it, and will respond promptly to questions about their projectors. The ones with 2K projectors don't advertise it and will ignore you if you send them a question about their projectors.
We also know from the official projectionist letter that no 4K version was released to normal theaters:
Here are examples of the 2D file name when a movie is released in 4K:
It is possible for a movie to be mastered in 4K and for normal theaters to only be given 2K versions. That happened both with Black Panther and with Star Wars 7: The Force Awakens. And it is possible for IMAX and Dolby Cinema to have 4K versions in those cases since they create their own digital files instead of using the files distributed by Deluxe. But if Infinity War had been mastered in 4K it would almost certainly have been listed on the realorfake4K website.
If you want to read projectionist letters for future movies, they are released on these sites:
Sometimes a studio is super secretive about letting the public see the projectionist letter for a movie, but they are publicly available for most movies.
I'm sure that IMAX theaters did show it in the 1.9 aspect ratio that it was filmed at however.
It wouldn't have done me much good, IMAX-wise, if they had mastered Infinity War in 4K. Most IMAX theaters only have 2K projectors. Only the Laser IMAXs have 4K projectors, and there are only about 20 of those in North America (none are in Michigan):