Nell, by my logic, you got it right. They take the waitress literally when she asks if they ALL want coffee. If the first one didn't want coffee, he would know that they then couldn't ALL want coffee so he would say no. It is only if he wanted coffee that he would have to answer, "I don't know" because he wouldn't yet know if every one of them wanted coffee.
The same goes for the second and third logicians until finally the fourth can say with certainty that they don't ALL want coffee because he doesn't.
Be kind, for everyone you meet is fighting a hard battle.