If you have a PS4 or XB1 console then I would reccommend a TV that has at the very least a 100Hz and preferably higher refresh rate. Yes you will pay a llittle more but if you like fast paced games then you wont regret it.
Picked that out because it's one of the issues right now. It get's harder and harder to know what your panel actually does. At the same time we get ridiculous CMR, CPI, blablabla numbers that actually tell absolutely nothing.
Just as a side note: I still have a 2012 well known brand TV. Calibration options it gives me, actual panel quality and the backlight of the not so super slim frame dadly let it have a better picture than many current, more expensive TV's... And yes, it has a 100/120Hz panel.
4K TV's don't even support half of the UltraHD standard. So if you're not sitting very close to your tv i'd effectively recommend a FullHD over a ceap 4K. Only reason to go with 4K to me right now, better models usually have 4K right now. I'm not talking about super cheap 4K right now.
Does the cheap tv even have HDCP 2.2 and so on? Does it have decent scaling? Otherwise you could have way worse picture quality.
Note: 4K is actually a cinema standard with some differing resolutions. 4K2K actually being 4096x2160.
'4K' UltraHD TV, also known as Quad Full HD or QFHD is 3840x2160
A 21:9 tv could have 5120x2160 pixels, that resolution isn't supported by the UHD standard though.