tvOS 15.4 can be downloaded over the air on the Apple TV through the Settings app by going to System > Software Update. Apple TV owners who have automatic software updates enabled will be upgraded to tvOS 15.4 automatically.
Apple’s tvOS updates are typically more minor in scale, focusing on under-the-hood bug fixes, performance updates, and small tweaks rather than major outward-facing changes, but there are a couple changes in tvOS 15.4.