FYI I purchased a 4k tv and returned it. There wont be a great deal of 4k content out there for quite some time. Also streaming such high resolution is going to require high speed data if you want clarity.
I would at-least wait until hdmi 2.0 is supported on the 4k tvs which only one model does currently. Cable will not be 4k anytime soon, Im not a huge movie junkie, I watch more pre-recorded shows then anything ( cable). I am much happier with a 65" samsung then a 4k 55" lg for the same price. Dont get me wrong if you can splurge on the sony 4k tv that's the way to go.