All signs point to a 4.7-inch iPhone shipping soon, followed by a 5.5-inch iPhone a few months later. This would be the first time Apple announced two different iPhones in the same year, as long as you don't count the iPhone 5c, which was basically a cost-reduced, repackaged version of the iPhone 5. The features of the iPhone 5c weren't new, whereas these new iPhones will certainly offer larger screens and perhaps more features like a faster processor, a better camera, and longer battery life.
So if Apple is to introduce two significantly new iPhones in the same year, perhaps they will break from the numeric naming method, as they did with the iPad Air. I could see Apple calling the 4.7-inch model the iPhone Air, since it will be the thinnest, lightest model (of this generation or potentially any iPhone) and the larger, presumably heavier 5.5-inch model could be branded the iPhone Pro.
This would give Apple several advantages. There could be a marketing campaign to promote the iPhone Pro as the tool for the business user, serious social networker, or hardcore gamer. The iPhone Air would be slim and light, and ideal for the fashion-conscious user.
If Apple doesn't announce both phones at the same time (which seems likely, given the rumors of a delay for the 5.5-inch model) then this naming mechanism works. Otherwise, there would be an iPhone 6 and an iPhone 6s for example, and the early customers of the iPhone 6 might get annoyed if they thought that their model was now obsolete. But by positioning them differently, they can continue to sell both as they do with MacBook Air and MacBook Pro laptops.
So the question we have to ask ourselves... are we iPhone Pro users or iPhone Air users? I use a MacBook Pro, so maybe I already have my answer!