Apple introduced the fourth generation iPod on July 19, 2004. The 4G iPod incorporated a click wheel, something that had been introduced with the iPod mini. This eliminated the need for 4 separate control buttons.
Unlike earlier iPods, the 3G iPod no longer had button surrounding the scrollwheel. Intead, there were four control buttons located between the scrollwheel and the screen.
The new iPods were more energy efficient and managed 12 hours of play from a 600 mAh battery. They were also slightly thinner than the 3G models.
The 4G iPods were the first full-sized models that could be charged using USB 2.0.