Background: I own a 65 inch Samsung 4k UHD with HDR also a Samsung 4k player and the 4k upgrade on netflix. Also a 60 1080 3d vizio in another room and a 40 inch 1080 samsung in the bedroom.
Just my 2 cents here...
Yes there is a difference but you have to know what you're doing. Going to Wal Mart and simply buying a "4k" for the sake of 4k may be disappointing. Not trying to be snobbish about it but this ish is starting to get complicated and you should really do your research before investing in one. My friend did this with a 4k non HDR Vizio, and to be honest it's not impressive whatsoever. Id take a good 1080 samsung all day over it. So if you're going cheap, stick with 1080 but get a great 1080.
But for the most part If just watching cable...the 4k will show an improvement over a 1080 tv. Yes, the broadcast will likely be in 720 but with a 4k, you're getting a better tv, period. It will upscale better, refresh rates are better ,color, etc. Both my 1080 vizio and Samsung look great by themselves but like garbage side by side with my Samsung 4k (I've tried this).
Most will inherently upscale blu rays as well and if you have a HDR tv paired with an HDR 4k player, you'll be blown away. HdR movies like Mad Max, Revenant, and the Martian are unbelievable.
Netflix 4k is amazing as well as long as you have the 4k netflix upgrade and are getting at least 25 mbps down or better on your home network. Several shows on there look absolutely stunning tbh.
So yes, theres a better picture out there to be had but do your homework and of course be willing to pay for it.
Additionally: Do not...I repeat Do Not purchase a non HDR tv if later planning or even thinking about going with 4k HDR movies.
I bought a $500 55 inch 4K tv from Target and 4K content looks amazing, noticeably different from 1080p, which also looks great. I'm sure the OLED looks even better but for $500 I'm more than satisfied. The only time I notice it isn't top of the line is very dark scenes, it does bleed a little. But everything else is stunning. So I don't agree about the 1080p thing, buy a $500 4K tv instead of $400 1080p all day.