The iPhone 4 was quite a change from the 3GS. The sleek and sexy curved case turned into a more mature rectangle arrangement. But with the big redesign also came upgraded hardware. For example, the iPhone 3GS had a 480x320 display. The iPhone 4 doubled that with its Retina screen.
Apple's Retina display is capable of a ridiculous resolution of 960x640. That's why everything on your iPhone 4 looks so sharp. If some of your app icons still appear pixelated or not as sharp, you may want to check this out...