A:AnswerIt 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.
A:AnswerAs far as counting your steps? I just tried it and yes it will count your steps. But it will not take your heart rate because it has to have skin contact.