With respect to speed, you can find little difference as 3G means third generation and 4G is a 4th generation. As the trend changes, if one service provider is giving 4G and others are giving only up to 3G then people will be more attracted with unique or more advanced features. In that case, all service providers will have to move to the same enhancement if they want to survive in their field. Once these features are in picture, the device should be compatible enough to allow them. There are some people who are still using the mobile phones which support only 3G, for them 4G will not make much difference as the device is compatible with 3G and the speed will be good too.
