GoKawiilYour TV will only output what's being sent from the connected device. The best streaming devices support 4K, but some older or cheaper models only go up to 1080p. So if you're using a gaming console, streaming stick, or similar device, check its settings to make sure 4K output is enabled. Be sure you're using a modern HDMI cable that supports 4K, as well.
With most streaming services, you must subscribe to the highest tier to access 4K streaming, but may still need to enable this option. For example, for Disney+, go to the main Settings panel, then App settings. There's an option for data usage, under which you can choose Automatic to "stream in the highest quality available" (up to 4K UHD, which uses around 7.7 GB/hour). There are also Moderate (HD) and Save data (SD) options. As long as Automatic is selected and you subscribe to the appropriate plan, you should be streaming in 4K when available.
When using a service that lets you change the quality on-demand, like YouTube, set the quality of the current video to 4K (though not all videos are uploaded in 4K). Live TV services typically include "4K" in applicable channels' number labels, though this has caveats. Such channels may be lower-res content that's upscaled instead of native 4K. Live TV rarely streams in true 4K outside of special sports events.
Virtually all Apple TV originals are in 4K, but not every show or movie you can stream via Apple TV is. You'll see the available resolution in the information list under the title when you select it. Ted Lasso, for example, is available in 4K, but live sports may not be. Amazon Prime Video has 4K on offer, but it's now locked behind a higher-cost Amazon Prime Video Ultra tier that removes ads and adds 4K access for available titles.