With the latest 4.3 update of the iOS environment, it is now possible to use your non-jail broken iPhone 4 as a 3G modem. This can be practical when your Internet connection isn’t working or if you are not at home (with a laptop but no Wi-Fi network available).
To do this, the procedure is very simple. You simply have to go into the "Settings" application and find the "Connection sharing" option. From here, you simply have to drag the cursor to "Connection sharing".
As for your devices, they will be connected thanks to the Wi-Fi network, Bluetooth or USB cable, with a password required if the connection is made via a wireless technology.
Note 1: verify your mobile phone contract beforehand so that you don’t have any nasty surprises at the end of the month. Note 2: the iPhone 3GS is also capable of offering this function, but only one device can be connected at a time. If the phone is jail broken, this limit can be lifted as it is not a technical limitation, but rather one imposed by Apple to force users to purchase the later model phone.