I don't know. I use two hands on my iphone all the time. Ever pinch to zoom? Prolonged browsing just using one hand get's tiresome very quick. I'm always shifting my grip/fingers.Have any of you actually used a phone with a 4" screen?
It's inconvenient as a phone, always have to use two hands or shift your grip to select something at the top of the screen. I have big hands and as it is I sometimes stretch to select the top of my iPhone screen.
If I wanted to use two hands with a PHONE I'd buy a samsung tablet.
Seriously though if you're so concerned about screen size just get an iPad.
Now for something like a phone call, you want to be able to do it all with one hand and that's just good UI design. I doubt an additional 3/16" of an inch would adversely affect that in a good design.
And please stop equating a 4" cell phone screen with a giant screen like the iPad. NOT the same thing.