tvOS 11.3 (build 15L211) is an official Apple over-the-air update for the Apple TV 4 (2015), released on 2018-03-29T17:05:12Z. The update package is 762.17 MB. This is a delta (partial) update that requires 10.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: f40038c98025bdb43b428d64c68dc74c29a01f73.zip.