Looking at a diagram comparing how easily a thumb can cover both today's 3.5 inch screen and the expected 4 inch display, you can see that a half inch makes a huge difference adding more of the yellow-colored "reach" area. A while back, when people were questioning Apple's decision to keep a 3.5 inch screen on its iconic smartphone in the face of ever growing Android models, it was determined that the Cupertino based company was more interested in keeping its customer's thumbs within easy reach of the entire screen. So the real question has to do with customer experience.
increased viewing space more than make up for the extra effort when touching the nether regions of the screen? This is something that Android users have had to adjust to and in the end, we see two of the platform's larger screened models, the Samsung Galaxy S III (4.8 inches) and the Samsung GALAXY Note (5.3 inches) both selling like hotcakes. And no Android user we know has suffered a sprained thumb or has complained about full-screen coverage.
If there is a difference between Apple iPhone users and those who sport an Android model, we will find out after the launch of the next iteration of the Apple iPhone. And then Apple will have another tough decision to make. If the new screen is universally accepted, does Apple go even bigger next year? And if iPhone users just don't like the larger screen, does Apple revert back to 3.5 inches? Actually, knowing Apple, the answer would be to keep growing the size of the screen on its smartphone to compete with Android and then offer the Apple iPhone mini, stuck at 3.5 inches for those who can't adapt to change.