The main difference between 3G and 4G mobile broadband is the data transfer rate. The 3G standard requires a transfer rate of 200 kilobits per second, while 4G has a rate of 1 Gigabit for stationary use, or 100 megabits for use while in a car or train.
A 3G phone cannot use a 4G network, but a 4G phone can use a 3G one. Network availability depends on a user's mobile service provider. In general, 3G speeds are adequate for regular data usage, such as e-mail or SMS. However, people who want to access streaming music or video on their phones should consider switching to a 4G-capable device, as streaming content requires high data-transfer levels.