Can you use a Bluetooth connection between iphone 4 and a computer with a internet connection to access the internet on your iphone? Wi-fi and cellular connection turned off or not available.
The iPhone only supports headsets and handsfree bluetooth profiles.
To use the bluetooth as your Internet connection would require the device to be jailbroken and have additional bluetooth profiles installed. You would probably also need additional software changes made to allow it to work this way.
I personally cannot imagine that someone would spend the time to make this work as it makes very little sense. Wi-Fi is simple and WiFi routers are cheap.