iOS 6.1.3 (build 10B329) is an official Apple over-the-air update for the iPhone 4s. The update package is 28.82 MB. This is a delta (partial) update that requires 6.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: e1b7f38c8001e20ccc706a5b1177217c19540318.zip.