The iPhone 4 Retina display is remarkably crisp and sharp. Still, if you read lots of email or send a lot of text messages on your iPhone, the small text can be quite taxing on your eyes.
iOS 4 let's you increase the size of text displayed in iPhone's Mail, Messages, Contacts and Notes apps.
Open Settings and then tap General. Tap Accessibility. You'll see a number of settings. Tap Large Text and choose a text size that's easier for you to read.
Now, when you read or compose email, send a text message, look up contacts or take notes in the iOS 4 apps the larger text size will be used.