iOS 9.2.1 (build 13D15) is an official Apple over-the-air update for the iPhone 4s, released on 2016-01-19T17:59:40Z. The update package is 901.45 MB. This is a delta (partial) update that requires 8.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: abb0203e29f9c9fc646c35247712f61faadffb08.zip.