The "mastered in 4k" blurays are just a marketing thing. They are NOT actually 4k resolution. They are 1080p just like any other bluray. The 4k tag is specifically referring to the fact that the film was scanned at 4k prior and then that master was scaled down to 1080p and put on bluray. 4k discs and 4k players do not exist yet. At least, not in a form available to purchase by normal consumers.
As for 4k TV's, unless you're buying a REALLY REALLY REALLY huge tv, you probably don't have the eyesight to actually take advantage of the difference. And by huge, I mean so big it won't fit in most people's houses. Like, movie theater sized screen big.
There are other things that are actually far more important than pure resolution in determining over all picture quality. These include black levels, actual demonstrated contrast ratio (not the gimmicky number they list on the box), color reproduction, and motion reproduction. There's a guy on another forum I frequent who works in the home video industry and he's posted links to various industry shoot out tests done for professionals showing this. One test even showed that the vast majority of people will tell you a higher contrast 480p image with good black levels is higher resolution than a low contrast 1080p image with crappy more-gray-than-black black levels. In other words, they actually perceived the lower resolution image as having more detail than the higher resolution because the other factors were actually more important than the resolution. The industry has just latched on to the resolution number because it's easy to explain and market. Now, that's not to say resolution isn't important, it's just that it's not the MOST important aspect of determining over all picture quality.
Personally, I would hunt down the biggest Panasonic or Samsung 1080p plasma I could find before they become impossible to get. Panasonic hasn't made them for at least a year now, maybe more. And Samsung is winding down production if they haven't stopped altogether already. But as of a few months ago you could still get samsungs new in the store. For Panasonic you'll probably have to find one on the aftermarket. Personally I think Panasonic plasmas are a little better than samsungs but they both consistently score very highly in professional reviews.
It's sad to see them not being produced any more because plasmas still consistently deliver the absolute best picture quality of any modern display technology. The only thing that can really compete with plasma technology is OLED, but it's still crazy expensive right now. CRT was also as good or better than plasma, but you're talking about a tv that weighs hundreds of pounds (no exaggeration!) for something like a 60 inch tv. And I don't think they even ever made CRTs that big. I think the biggest were in the 30 to 40 inch range.