Just saying an iPod work with the 4G iPod does not mean that it will work with all the different iOS for the iPod. Sometimes an iOS update will beak compatibility.
Is the iPod fully seated in the dock?
Sometimes this works:
- Reset the iOS device. Nothing will be lost
Reset iOS device: Hold down the On/Off button and the Home button at the same time for at
least ten seconds, until the Apple logo appears.
- Reset all settings
Go to Settings > General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Information (such as contacts and calendars) and media (such as songs and videos) aren’t affected.
- Restore from backup. See:
iOS: How to back up
- Restore to factory settings/new iOS device.
- Make an appointment at the Genius Bar of an Apple store.
Apple Retail Store - Genius Bar