tvOS 11.3 (build 15L5211b) is an official Apple over-the-air update for the Apple TV 4 (2015), released on 2018-03-20T20:06:27Z. The update package is 55.9 MB. This is a delta (partial) update that requires 11.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 24c9e0932a43cd60a900a0ae30a9cc5a8a665e65.zip.