There is no iOS 4.1.3. If it's using iOS 3.1.3, 4.2.1, or 5.1.1 and iTunes says that no further updates are available, it's running the highest iOS version it can.
If there are no updates available then that means your iPod is too old to update. It's probably a 1st gen (maximum iOS 3.1.3) or 2nd gen (4.2.1). No way around this.
If it is not, then...