You can try to book them on the Austrian Railways website, oebb.at
It is not the best designed website ever, but here it goes:
- input date of travel, time (choose 9PM) and VENICE and MUNICH
- hit Book Ticket
- choose "one-way ticket and day ticket"
- now, at the top, next to VENEZIA and MÜNCHEN you should see "who is going?": click "Change"
- add the 3 other passengers and for each passenger, click "Add Discount" and search "Eurail"
- then hit Next at the top right, and at last you can look for the direct "NJ" train and see if Couchettes are available...
But for next week, there is a non-zero chance that 4 couchettes won't be available. Fingers crossed!
If you end up unlucky, there seems to be an afternoon daytime connection that isn't too bad: leaving Venice around 3pm and getting into Munich shortly after 10pm. You still need a reservation for it, but it only costs 10€ vs 44€ for the couchette, so you would recoup a bit of the hotel costs in Munich.
As for German and Dutch trains you will not need reservations with a valid Eurail pass, so no need to worry, just hop on any train. You might not always find 4 seats together for long-distance high speed journeys, though: in Germany, the Deutsche Bahn website (bahn.com/en) lets you purchase reservations quite easily , just choose "seat only (no ticket)" on the front page.
In Dutch domestic trains, reservations do not exist.
