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If you've just bought a 4K UHD TV, you're probably eager to stream some 4K content (especially if you don't have a UHD Blu-Ray player or any way to watch 4K content, otherwise). Since almost every modern 4K TV is a smart TV, it should be as simple as plugging it in and booting up YouTube.
While there are many ways to watch 4K content, if you want to stream it, you're going to need a pretty good internet connection. Early reviews of 4K streaming would claim you needed a "robust" one, but generally, as long as your internet connection isn't dial-up, you should be fine.
While you can run an ethernet/category cable to most mid- to top-tier 4K TVs these days, you shouldn't need to in order to stream 4K content. Different streaming services claim different minimum requirements, but generally you should be able to stream 4K content from any provider (Netflix, iTunes, Amazon Video, etc.) as long as you have at least 25 Mbps WiFi (down).
To check your WiFi speed on a smartphone or laptop, head somewhere like SpeedTest and run the (relatively quick) diagnostic. While this test isn't perfectly accurate, if you're sitting right at 25, you probably won't be able to stream 4K content. You can cross-check at TestMy.Net if SpeedTest isn't working, too.
However, unless you have the absolutely cheapest WiFi plan available, I'm going to bet you have more than a 25 Mbps plan. Keep in mind however that if you do want to stream 4K content on such a plan, you'll need to kick every other device off of your WiFi to ensure you're getting all the juice to your TV. But if you're streaming 4K content to a fancy 4K TV, it might be time to upgrade your internet, too.
If you're paying for 100 or more Mbps from Comcast, Cox, AT&T, or some other provider and are getting buffering/compressed 4K content, something isn't transferring correctly. While that's something you should probably contact your ISP about, it's also worth ensuring that your router/modem are on your ISP's approved list of devices, and perhaps to do the classic unplug-it-for-10-seconds maneuver.
If you suspect the issue is on the TV's end, you should be able to use the Network sub-menu to check your TV's internet status. Netflix and some other streaming options will also often tell you at what resolution your content is streaming if you enable that information. Just keep in mind that sometimes, 4K video takes a few seconds (or sometimes minutes) to buffer up to full quality. It may sit at 720p or 1080p for a few, but if your internet is working, you should expect to get 4K resolution streaming to your TV without too much hassle.
Coincidentally, you may want to update your router: check out our list of the best wireless routers, or consider using a mesh WiFi system to add some robustness to your home network. Finally, if you're comfortable running an ethernet cable to your TV (or streaming device) to ensure that your internet is robust enough to stream 4K content, or want to boost the WiFi signal within a chain, you may want to consider using a powerline extender or powerline adapter.