iOS 8.4.1 (build 12H321) is an official Apple over-the-air update for the iPhone 4s, released on 2015-08-13T18:05:12Z. The update package is 155.79 MB. This is a delta (partial) update that requires 8.3 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 0d4fdf1733ce3b3dc6768ca6cca01a86ed4eb0dd.zip.