As mentioned a few times in the blog there is no 4K native material to even understand its full advantages or disadvantages at this time. It is all theory. Right now, your TV's and Bluray players claim to support a "4K medium" that doesn't even exist yet, but only upsamples/upconverts to what they think it may look like. This is similar to an external DAC that has upsampling. Your TV's and Bluray players don't support it currently but the "upsampling/upconverting" acts as an advertisement of what it could be if/when there is a true 4K native format and devises that can actually support it.
Hollywood and others will have to get onboard and install the new 4K hardware/software into their cinematic equipment in order to shoot movies, commercials, sitcoms etc. in real native 4K resolution in order to see what it will look like without "upsampling/upconverting.
People who claim to have watched 4K material have not watch 4K material as it does not exist at this time.
While it's still early days, the prior comments were slightly misleading, and the last statement seemingly false. 4K is just getting out of the gates, and with all the investment Sony & others are putting into it, wider implementation is not that far down the road. E.g. 4K mastering movies by re-scanning negatives in 4K, shooting popular series in 4K (e.g. breaking bad), etc.
Yes - most digital movie theater projectors are still 2K - though the upgrade to 4K is in progress (e.g at AMC). Now whether for those theaters that have done the 4K upgrade are getting their digital movies @ 4K : don't know. But presumably it can't be that far off.
As for people claiming to have seen 4K material not having seem it as it doesn't exist.........any Magnolia Design Center has 84" Sony 4K sets connected to 4K media servers displaying 4K native demos. Yes, in some cases they'll put spiderman or whatever that are "mastered for 4K", but other demo material, the short clips that are often playing, should be 4K native. So not a very high bar for folks who want to see it : MDC's are not @ all BB's, but finding a BB that has an MDC reasonably close should not be that difficult.