iPadOS 9.1 (build 13B143) is an official Apple over-the-air update for the iPad 4 (WiFi), released on 2015-10-21T17:15:52Z. The update package is 944.7 MB. This is a delta (partial) update that requires 8.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: b2f21f2a0cd9f334bc2d7b3e0013b40d28d2bba8.zip.