Next iPhone will be the iPhone 5. Bet you a buck.
See a trend here?
1st iPhone's name: iPhone
2nd iPhone's name: iPhone 3G (3G being the new model name)
3rd iPhone's name: iPhone 3GS (3G being the model name with an S for midcyle spec bump)
4th iPhone's name: iPhone 4 (4 being the new model name and debuts with iOS5 - whodda thunk?)
5th iPhone's name: iPhone 4S (S for midcyle spec bump)
6th iPhone: iPhone 5 (5 being the new model name)
7th iPhone: iPhone 5S (S for midcyle spec bump)
Apple has pretty much established a pattern here. Those who think the next phone will be called the iPhone 6 are daft or trolls.
Based on that pattern, the next iPhone is just as likely to be the iPhone 4G. All that pattern indicates is that the next iPhone won't have an "S" at the end.
3G was an indication of the new data network support. The "4" was the only iPhone to be named after its generation.
If they were just basing it on case redesigns (as you put it "new model name"), than the iPhone 4 would have been the iPhone 3!