I just bought an iPhone 3G secondhand from a friend. Under Settings the version is listed 4.2.1 but it's missing some of the iOS 4 features. Double-clicking the home button doesn't bring up recent apps, and there's no option to set this function in the home button settings. There's also no Game Centre on the home screen. Does anyone know why this might be?

EDIT: While it will download apps requiring iOS 4.0 from the App Centre, they crash immediately after opening. Apps requiring iOS 3.0 work fine.