The iPhone 4 will hit the market Thursday and a wave of excitement has washed over techies, current iPhone users, and prospective buyers looking to see what the latest arrival will hold. As it stands, the iPhone 4 comes with a bigger battery, giving users 10 hours of Wi-Fi Web browsing and seven hours of 3G calling. Sleeker than its predecessor with flat front and back glass surfaces, the iPhone 4 looks like a strong contender in the increasingly competitive world of smartphones. But are the differences enough to make customers switch? Take a look at these key changes.