If Apple sticks to their current pattern, there'll be an iPhone 5S later this year, and the iPhone 5 will drop to $100 on-contract, the iPhone 4S will drop to free on-contract, and the iPhone 4 will be retired. If Apple sticks to their current pattern, there'll be an iPhone 6 in 2014, and the iPhone 5S will drop to $100 on-contract, the iPhone 5 will drop to free on-contract, and the iPhone 4S will be retired.
But Apple doesn't always stick to their current patterns...