Yeah, that probably accounts for the majority of viewers. My wife and I have a small computer in our living room hooked to a 42" television so both of us can enjoy the absurdities of the Internet from the comfort of our couch. And also so she can preview her YouTube videos before making them public.
Regardless of screen size, when it comes to YouTube, the real issue is bitrate more than resolution. A 1080p video on a 24" monitor can still look bad especially during high motion sequences if the bitrate is too low. And YouTube’s default bitrate is kinda low.
You’re already aware that a 4K video uploaded to YouTube gets transcoded to 2160p, 1440p, 1080p, 720p, 480p, 360p, 240p, and 144p. The great thing about uploading in 4K is that all eight transcoded versions get higher bitrates, not just the 4K version. So, to get a better looking 1080p on YouTube, it pretty much has to be uploaded as 4K to trick YouTube into thinking this video is extra special and worth the higher bitrate.
I used to have that problem. After reading a lot of other people’s experiences and experimenting on my own, I concluded that YouTube isn’t so much messing up the colors as it is making horrible guesses about what the colors are if the smallest bit of color metadata is missing in the uploaded file. My problems went away when I fully specified color space and color range in an MPEG-4 container, which is the container most likely to be interpreted correctly due to sheer popularity. I think I recall that you preferred to upload HuffYUV files. That codec would obviously require Matroska in order to capture both color space and color range given that AVI doesn’t have those flags. If Vimeo works and YouTube doesn’t, it probably comes down to Vimeo making smarter guesses about the missing (or ignored) color metadata.
Sure, here’s a video where my wife made a 3-foot model of Downton Abbey out of gingerbread. The big reveal is at 9:15 if you want to skip to the finished build:
This video was uploaded as a 4K MPEG-4 with H.264 at CRF 16 (Shotcut quality 68%) and was 11.2 GB in size. For kicks, I made a HuffYUV version, and it came out to 185.6 GB with zero discernable difference in video quality after doing A/B tests between the two on a Shotcut timeline. I love the idea of lossless codecs for masters, but my pocketbook cares more about the cost of hard drives for archiving. H.264 CRF 16 has become my sweet spot between the two concerns.