High resolutions like UHD require really high bitrates when encoded with H264, because H264 wasn't designed with such high resolutions in mind. HEVC/H265 improves upon its predecessor in this regard, so Netflix's edge hardware only keeps their UHD content encoded in HEVC/H265. If you request HD content, you're not only getting a different resolution, but also likely a different encoding scheme.
Chrome on Windows has Widevine support but not the level required for providers to serve 4k content to it. As mentioned in another comment it's also possible the 4k content is only served via HEVC, so lack of Widevine support in HEVC would mean there is no compatible 4k steam available.
Widevine, and DRM more generally, has everything to do with why most providers don't stream above 1080p(even if) on Windows. Netflix's Windows app (ie not Chrome and possibly limited to only Netflix Originals..) being a notable exception:
Amazon Prime Video: Nope
HBO Max: Nope
Paramount+: Nope
Disney+: Nope
Hulu: Nope (Maybe originals in the app?)
Apple TV: God nope; ugly as sin I don't even think they are hitting 1080p but maybe I'm just spoiled now
There may also be aggressively limited bitrates at 720/1080p for H.264 streams. Would need to dig into it but that could make those resolutions look like garbage compared to how they used to look(ie a Vudu 1080p stream circa 2011).
Basically it's a shitshow and a damn shame for personal computing in 2022.
