UNICORN! AFAIK, they do not exist yet. There are essentially no 4K displays on the market and receivers with the matching technology usually follow a year after that tech comes out, so I would stay go with something current or wait. I personally don't feel 4K will be the direction the industry goes. Since all the manufacturers are still sort of in a wait and see pattern, I don't see it happening anytime soon.
BTW, not unlike computers, some of the features will ALWAYS be obsolete within 5 years on a receiver. The standard audio functions pretty much don't change though, so focus on audio over features and you should be fine. My receiver is around 10yrs old and it still works with current technology - there's almost always more than one way to get things to work.