I know Apple dislikes cluttered product lineups and I do as well, but three models for an as personal device as the iPhone really makes a lot of sense. In my opinion, 5.5" is "XL", 4.7" is "L", and then... Well, then that's all you've got. Re-introducing size Medium (or Small if you like) just makes a lot of sense in this case.
If they did this, they could move on without touching the display sizes for a LONG time to come. I can't really imagine they would then need to change them for the foreseeable future. I could before the iPhone 6, but not if they did this, which is great for developers and helps with confidence in the new lineup.