I can assure you all without a doubt that the phone to the right is a 4" phone, assuming the phone on the left is a 3.7" Defy. I took the image from the original post and used photoshop to make some measurements.
I drew a diagonal line across each phone to get its dimension from the picture. I duplicated each line and copied it on the left side vertically so I could measure it on a scale. The Defy came out to 5.65625, which we know is 3.7". The Bionic came out to 6.21875. Via a little math, that makes the Bionic 4.0679" - might as well round it down to 4". There you have it folks, see the image below.
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