What I meant is that you need a certain amount of pixels just to be able to fully represent the 1080p video pixels, and if you see "extra space" in the VR world around the screen, then you need 1080p video + extra space in pixels, so it probably should be at least 4k.
You need a certain amount of pixels to represent the 1080p video in one particular frame. When the next frame comes along, your head has moved enough that you'll be seeing a different subset of those total pixels. At sufficient framerates (and motion capture rates), this actually does a pretty good job of approximating the full resolution of the imagery (especially when this is happening 2 or 3 times per source video frame, as the case might be with NTSC/PAL content).
