If you have an iPhone 4, 4s, 5, or 5c, you can download the Stridekick app (just make sure your phone is updated to iOS 9.3 or later) and connect any leading fitness tracker.
The iPhone 4, 4s, 5 and 5c cannot connect as your step tracking device because these models don't have the part built in, the accelerometer, that tracks steps (this is included in the iPhone5s and newer).