The iPhone 4 was a major step forward over its predecessors in a number of areas. First, the iPhone 4 is visibly different from earlier versions thanks to its more-square shape (gone are the iPhone 3GS' tapered sides), a microSIM slot on its side, and circular volume buttons on the left. When using the 4 it was also immediately clear that something had changed: the screen was much higher resolution; it was the first iPhone to use the Retina Display screen technology.
