My family of four plans to take the night train from Munich to Venice in early June, 2013, and we would like to make reservations as early as possible. We have 3-country eurail passes (Germany, Austria, Italy). On the CityNightLine website, I see where I can reserve bunks in a 6-bed couchette (although it's too soon to be able to reserve for our specific date). But I don't see the option of paying a little more to reserve the entire compartment, so that we don't have two other people in the compartment with us. Is it possible to do this? If so, how?
You can do this on Bahn.de. Put in 4 travelers in the search and look for the 4-person Abteil in a Liegewagen. I tried a closer date (in April) and it showed a cost of 150 euro each with Eurail passes.
There is a direct Eurocity (EC) train that leaves Munich at 11:31 and gets into Venice at 6:11 PM. You might consider that instead and save the accommodations charge for a room in either Munich or Venice. Plus, you'll be awake for the trip over Brenner Pass.
As Lola said, you can book all the bunks in a 4-person couchette for a total of €150 (€37.50/person). If you were to book all the bunks in a 6-person couchette, it would cost you €165.