There is no service that offers all content in 4K or 1080p quality, including Comcast, DirecTV, or Dish. While the connection of the box to the TV may always be 1080p or 4K, the box has to up-sample a lot of the content from the provider. For example, if you are watching an old episode of Seinfeld, 1080p (and surely not 4k) just didn't exist back then, so whether it's upscaling to 720p, 1080p, or 4K will make no difference in the picture quality. The Fire Stick we install is a 4K capable device. If you have a 4K TV and are watching 4K content (some Amazon shows are in 4k, and Pluto has some free content in 4K), and you are not processing the video through an older receiver that can't do 4K it should all work.
Using YouTube TV or Sling, you should find most of the major prime-time shows are available in 1080p. Most daytime TV is not (this applies to Comcast, YouTube TV, Sling, etc). YouTube TV and Sling will also adjust the quality based on your internet speed, network speed, and demand on the content providers servers. That said, if you want to force YouTube TV to provide 1080p picture when available, you can change the YouTube TV settings to force 1080p. If the show is available in 1080p, it will list 1080p there and you can select that instead of Auto. Still, you will be limited by the content resolution. Even if you select 1080p, it can't make a program that is not 1080p content look better.
To change the video quality in YouTube TV: While watching a show, press down twice on your remote, then go left to the three vertical dots. Go down and select Video Quality and select 1080p. If you only see 720p, that means that's the best available for that content.
To change the video quality in Sling: Go to: Settings (gear icon) > Connection > Best Quality
Just be aware that if you watch a lot of TV, streaming 4K content, or even 1080p content for hours and hours a day can lead to issues with data caps if your ISP has data caps. Most of our customers don't run into problems, but it is something to keep in mind.
Note that no Over the Air content (via antenna) is available in 4K... at least not until
ATSC 3.0 is available.
It's possible a TV is only connecting to the Fire TV Stick at 720p, and that can be a setting on the TV or on the Fire Stick. To check your setting on the Fire Stick, go to: Settings > Displays & Sounds > Video Resolution. This is normally set to Auto, which means your TV and the Fire Stick will "talk" and decide what resolution to choose, and will choose 4K if that's available. This is generally best, but if there's a problem with the communication with your TV, you can force it to use a specific connection resolution.