tvOS 12.1.1 (build 16K45) is an official Apple over-the-air update for the Apple TV 4 (2015), released on 2018-12-05T18:08:57Z. The update package is 753.65 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: 4c8c2dc89243c219dba664bd3d41c669cb0ccb4f.zip.