A:Answer It depends on the version of iOS you are running, not the model Apple device. Can the iPhone 4s run the latest version of iOS (currently iOS 9)? If yes, then you will be okay.
My wife and I are running iOS 9 on all of our Apple devices (iPhone 5 & 6s, and iPad mini 2 and air 2), and it works perfectly with all of them. So long as the iPhone 4s can run iOS 9, you are fine. If it does not run the latest version of iOS, then I am not sure if the app is compatible.