Check the compatibility list of i OS 9 you will find that i Phone 4s and i Pad 2 are also supported by i OS 9.
However, not all i OS 9 features will be available for them.
For some whose i Phone 5/5s/55c running with i OS 8 or i OS 7, they need to upgrade to i OS 9 firstly. Plug the i Phone to your computer via the USB cable and make sure that the latest i Tunes is installed. To keep copies of the whole i Phone/i Pad content on computer in a viewable format, you can try an i Tunes alternative like Any Trans to transfer photos, videos, songs, messages, notes, contacts, voice memos, and more to computer.
To help you make a smooth upgrade and stay away from all kinds of i Phone i Pad update issues, here in this article we collect the most common i OS 9 problems and solutions. Cellular data issue after i OS upgrade is also extremely common.
i Messages Failing to Send on i OS 9/9.2/9.2.1/9.3 /9.3.3 13. Even Apple revealed at WWDC that i OS 9 will only be about 1.3GB in size, it can still be a tough task for many 8/16 GB i Phone or i Pad users to free up that much space, especially if they're not syncing to i Tunes to manage storage manually.
i OS 9/9.2/9.2.1/9.3 /9.3.2 is Slow on Old i Phone/i Pad 10. Data Loss After i OS 9/9.2/9.2.1/9.3 /9.3.3 Upgraded 12. Or you can try again with these methods mentioned in this guide - How to Fix Software Update Failed Error, like, updating with i Tunes, upgrade with IPSW file, etc.
The new "Low Power Mode" of i OS 9 lets you extend your battery life to some degree.