How to Reset the Network Settings on an iPhone 4
With your iPhone 4, you can browse the Internet, check email and download apps quickly by connecting to wireless networks. Using the tethering feature, you can also share Internet connections with other portable devices such as laptops. If you run into iPhone network connection problems, or are unable to trace wireless networks within range, resetting your network settings might solve the issue. Doing so restores your phone's default connection settings.
Turn on your iPhone by pressing the "Menu" button. Drag the slider to the "Unlock" position to display the home screen.
Tap "Settings" to display your phone's settings, then tap "General."
Scroll down the "General" settings to the bottom and tap "Reset." Tap the "Reset Network Settings" option, then tap it again to confirm. Your iPhone should shut down for a few seconds, reset your network settings, and restart automatically.