Eddieg24 - If you only have 4 hours-then the ship's tour is the way to go. If you have 16 hours then I suggest you consider hiring a local driver at the pier. There is a great D-Day museum located at the location where French General LeClerc came ashore and is very informative.
If history is not your thing- A run over to visit Mt. St. Michel and tour therein (It is a medieval town with a monastery at its peak-the views alone you'll remember the rest of your days!) would be quite memorable and is priceless.
The French people of Cherbourg are wonderful and most hospitable.
There are enough remains of German fortifications around the harbor and the piers to write a book!