Taking a look at the image below we notice that the 4.7-inch device has flat sides while the larger device has rounded sides. One thing they both have in common is a new button which would be on the right side when looking at the front of the device.
Recent rumors have suggested that Apple would relocate the Sleep/Wake button to make one handed operation easier; however, that button still appears that the top of both devices. Thus, it's unclear what the new button would be for.
The 4.7-inch iPhone is expected in September; whereas, the larger 5.5-inch device is expected sometime before the holidays.
More details on Apple's roadmap can be found here.
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