YouTube downscales it to 1080p and I get to watch it in 1080 again.
Youtube doesn't downscale to 1080p, it transcodes to 1080p. I'm pretty sure you're supposed to watch the 4k version, even if you're not on a 4k monitor (which will be downscaled), to get the extra bitrate. Someone needs to download the videos and compare the bitrate of the 1080p version of the 4k upload to the 1080p version of the 1080p upload. One thing to consider is that when uspscaling during rendering, the 4k version will require more bitrate to not look like crap, so you need to subtract that bitrate overhead from the extra bitrate that youtube gives you on a 4k upload. 4k is 4x the data of 1080, but I am not a video expert and have no idea how x264 or whatever encoder you use handles compression of 1080 vs 4k. Is a compressed 4k test file 4x the file size/bitrate of a compressed 1080p file? I don't think so. It can be less, or even more, based on the content and compression. I am also not sure if 4k of actual pixels take up more bitrate than 4k pixels that were produced by an upscaling algorithm. It's very complicated to quantify the quality of such a test objectively. However, with some testing and math, you can come up with an estimate.
I cannot comment on the subjective quality as my monitor fell off my desk, so I'm without a display/computer
There are however other factors to consider. The hw acceleration implementation in chrome,firefox,etc isn't very good. Neither is software rendering. The people complaining about stutters in the comments need to realize that 4k playback requires a decent internet connection and a decent cpu, especially since the medium is a web browser.
One benefit with higher resolution sources is watching them on high resolution displays like many modern smartphones. The upscaling (1080->1440) in the youtube app and through a web browser isn't very good. I notice an immediate difference in quality watching a 1440 video on the native 1440 screen of my Nexus 6 compared to a 1080 source, even if the actual visual quality of the content is the same. One must appreciate the fact that the quality youtube delivers is amazing considering how much data Google has to store, process and deliver. But you must also acknowledge obvious limitations from an enthusiasts point of view.
I'm getting a professional 4k display on Tuesday so I can actually test everything.