It was waitlist for a while, not sure if it still is but I got my welcome email like a week later.

I believe 4K is already basically there. I have a 50" 4K (2160p) that I sit 9 feet away from and based on the Nvidia PPD calculator, that makes for 168ppd, and according to that page 150ppd is around the upper limit of human vision. Apple’s “retina” displays target around 50-60ppd (varies based on assumed viewing distance), which is what most people seem to consider “average eye visual acuity”. Imo 4K / 150ppd is more than enough.