I'm definitely on the Pimax 8K bandwagon.
It actually renders at 1440p per eye and then upscales to 4K. So the horsepower isn't as much as it sounds - made even more confusing by basically lying about the resolution in it's name.
But, from the hands on I've read and watched it sounds like screen door effect is all but gone and the 200 degree FOV is real and amazing. Obviously there is a lot more detail in everything as well.
The issues at the moment sound like it's more to do with head tracking (done using Vive's) and comfort. Also some screen blur which is far more of a concern to me as it uses LCD, not OLED, and I believe blurring is inherent in LCD which makes me think that is one issue which might be harder to fix.
Either way, 4K VR is what I'm waiting for.