2160 * 1200 (Oculus Rift CV1) 3 bytes per pixel * 8 bit depth * 90 FPS (Oculus Rift required spec) = 5.5 Gbps.
3840 * 2160 (4K) * 3 bytes per pixel * 8 bit depth * 90 FPS (Oculus Rift required spec) = 17.9 Gbps. At 60 FPS that drops to 11.9 Gbps. To fit in 8 Gbps you have to drop the framerate to 40FPS, which isn't really good for VR.
Yeah, it works for the CV1. But anyone who's used one knows that a higher resolution is badly needed, so obviously the next iteration will have to be better. I've been hearing talk of 8K not being enough for ideal performance.