Thanks for the info. I would say anything with ios 7 is still ok for now as it will take the current version of IG in the app store.
Is your 4s jailbroken? If not wait for the next jailbreak which will include 9.2 and look at some of the app clones you can get.
Cloning apps is only for jailbroken iphones but it gives you more than one instgram app on your phone.
(until IG gives us the option to switch between accounts...)