With the iPhone 4 Dock, you get easy access to a dock connector port for syncing or charging, and an audio line out port for connecting to powered speakers via an optional audio cable. The iPhone 4 Dock also works with the USB Power Adapter.
Charge and sync your iPhone 4 with the iPhone 4 Dock. Your iPhone 4 sits upright in the dock as it syncs or charges, so it's perfect for a desk or countertop. It features a convenient audio line out port that connects to powered speakers. It supports other iPad accessories, such as the USB Power Adapter. You can even conduct speakerphone calls with your iPhone 4 in its dock.

Here are some ways you can use the iPhone 4 Dock:

Connect the dock to your computer with the USB cable included with your iPhone 4 to sync iPhone 4 and charge its battery.
Connect the dock to an electrical outlet using the Apple USB Power Adapter (included with your iPhone 4) to charge the battery.
Place your iPhone 4 in the dock and enjoy clear audio during speaker phone calls thanks to special audio porting.