My advice is to get passports and get the books. Technically you only need a birth certificate and a government issued photo ID to cruise on an itinerary that starts and ends in the US, as a 4-day does. That said, should you encounter an issues while out of the country and end up needing to fly home you will need a passport.
We will not travel outside of the US without passports. We all have the books. The card is good for land and seaport entry into the US from Canada, Mexico and the Caribbean only. It is not valid for air travel. It will do nothing for you that a birth certificate and photo ID can't do.
Happy sailing!