If the older iPhone is still capable of operating on the carriers system they're likely to let you keep using it as you would be a source of revenue to them. With the iPhone 4, if the 3G system is ever shut down, that will be the end of being able to use it on a cellular network as the iPhone 4 is not LTE capable.
You're also limited to CDMA carriers as the iPhone 4 came in separate GSM and CDMA versions.