If they can't wait until 4k matures a bit more and 4k bluray hits the market, and they're determined to buy a new TV now anyway, I'd say go for it. Take em somewhere like a walmart, best buy, or preferably Fry's Electronics to check em out in person. If their eyesight is good, they'll most likely love and appreciate the difference in quality.
My family purchased a 1080p TV last year, picking one I recommended. If we were shopping in today's market, I would have definitely tried to convince them to go 4k. Sure, you can get 1080p for less -- but it may save the headache of having to buy yet ANOTHER TV in the future when 4k is more mainstream and media is widespread. Bluray 4k is likely going to be out later this year - so media availability, while very lacking now, will likely flood the market soon. (Just got back from Fry's and it seemed like there were more 4k than 1080p TVs!) .. I don't think it'll take long to hit critical mass, certainly will be quicker than it took 1080p due to the fact that HD was prohibitively expensive when it first came out and 4k is already fairly affordable.
I have a 4k 30" monitor now and was just drooling at the idea of having one of the 120hz /60hz Sony 55" 4k TVS as a monitor. Not that I don't absolutely love my 30" , the only problem is that it makes it somewhat painful to go back to using 1080p or less. It's easily one of my all-time favorite computer component upgrades.