iOS 9.3.5 (build 13G36) is an official Apple over-the-air update for the iPhone 4s, released on 2016-08-25T16:53:26Z. The update package is 857.1 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: 4dcd28daea2aa65399d7196e2cf8dd660cf6f9a3.zip.