Indeed.
If they combine full foveated rendering with simultaneous multiprojection, I wouldn't be surprised if something as low as the 4GB RX 480 could run a 4K VR headset.
4K only requires ~61% more processing power than the current VR resolution (since the actual render resolution is 3024x1680). And that would be if you used no software tricks to make it easier at all.
I'm hoping they go for 3x the current PPI/resolution, along with a 30% increase in FOV. The way screen manufacturing is looking, they should be able to pull that off relatively easily within ~12 months.
It would result in a resolution of 4858x2699 (or thereabouts). But would only require a screen PPI of 801, the screens would be physically larger to make the FOV bigger.
And even though that resolution may look too big to render, it is actually only ~2.6x the number of pixels already being rendered with VR headsets (so comparable jump as going from 1920x1080 to 3440x1440), and a GTX 970 can do the current tech. With a couple of software tricks (like foveated rendering) it should definitely be doable on a GTX 1070, and if they can do a lot of optimisation, even an RX480. Also since they've already got the dynamic supersampling in place they could subsample to make it easier to render.
The important thing is to get more PPI and more FOV into the headsets. You wouldn't actually have to render at the full screen resolution to make it gorgeous. You just need it to have very high clarity (can't see the physical pixels on the screens) and have enough FOV to be hugely immersive (they're already 95% of the way there anyway, so a 20-30% larger FOV would be more than enough).