I have some 4k mastered discs with me here. In around 3 weeks time I will be able to tell you if they look better on a Sony 4k projector. I can understand why people are skeptical, when you view them on a standard 1080p TV there is no difference in quality whatsoever. However, David is not the first person to mention this and I don't doubt him.
Now the interesting thing is - Are we looking at better picture because of the 4k mastering or is the upscaing to 4k taking effect? Perhaps both?
Only way to find out is by doing an A/B test, with a standard disc and 4k mastered disc. I have a few in both formats and will report back