what apps can be added?

if Youtube TV (or other TV streaming) apps are available for 4th or 5th gen?

Yes.

YouTube TV, Hulu Live, Sling TV, DirecTV Stream (formerly AT&T TV Now), FuboTV, Philo, all these internet streaming TV providers have an app for Apple TV (4th generation or later), as available from the on-device App Store, along with thousands of other apps.

A third generation or earlier Apple TV can only get applications through OS updates. Newer models can access the App Store.

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