iphone 5c could not be synced an unknown error occured (-4)
Launch iTunes, connect the device and uncheck Sync Music, Sync Movies, Sync TV Shows, and Sync Podcasts” in iTunes.
Click Sync. This will remove all media from the device but keep it in iTunes.
Disconnect the device from the computer.
Switch off the iOS device by holding down the Sleep/Wake button and using the  Slide to Power Off button.
Hold down the Sleep/Wake button to turn the device back on.
Connect the device to the computer.
Click on the device in iTunes and tick the media that you want to sync.
Click Sync.