The fifth iPhone will be called whatever Apple wants to call it. And from the announcement on that's what we'll all call it as well. No one calls the 3GS the iPhone 3. No one calls the iPhone 3G the iPhone 2.
Our speculations on naming conventions, no matter how rational, have no bearing on how we will be referring to the fifth iPhone in the future. It all depends on how Apple chooses to brand their next phone.