If your are a Windows user:
1. Go to Control Panel
2. Go to "Hardware And Sound"
3. Go to "Devices & Printers"
4. You'll see your iPhone as an unverified device. Right click on it's icon and select Properties.
5. Open Hardware Tab. You'll see a Properties button below. Click on it.
6. There are 3 tabs (General, Driver & Details). Open "Driver" tab. And choose "Update Driver".
7. And then choose "Search Automatically For Update Drivers". then install the Update Driver you searched.
8. Open itunes again & plug in your iPhone 4 again.
You'll see your iPhone 4 syncing again.