Vrsumo2017 VR video files don't use aspect ratios common in the movie world. There is no 16:9 in VR. So things like 4K or 8K don't have the same meaning as for 2D videos.
A 4K VR video consists of two videos (one for each eye) in a single file. So 3840x1920 actually is 2 x 1920x1920. At least the vertical axis is close to 2160, which is known as 4K in the movie world. The horizontal axis obviously isn't and needs to be stretched.
So companies just take the largest number (3840 in that case) and use that to define the resolution of VR porn.
The next step (5K) is the most meaningful one for improvement of the image quality, because 5K usually means 5400x2700 - almost twice as much pixels. Everything below 5K looks bad on Quest 2 IMO, but I haven't checked 4K files from current cameras - maybe they are slightly better.
