On the iPhone 4, 4S, and 5 dust cannot get under the screen. The LCD screen and the glass are fused together and dust cannot get in between. The only way that you could have dust is if it was present in between the two when the display assembly was made.
Don't leave your device in dusty areas. I've heard stories of people returning their devices because they've had little specks of dust trapped underneath the glass screens. But other than that, I don't think there's any way you can get rid of the dust yourself -- taking apart an iPhone can be a pretty tedious task.