Sorry, but I'm not sure what you mean - if they book a 4 day
Free Dining package that includes at least a one day ticket for each of them, they will get free dining based on each night of their stay. So say they check in on a Monday and check out on a Thursday - they would get free dining for:
Monday
Tuesday
Wednesday
They wouldn't get any credits for their check out day of Thursday (it's based on each nights stay) but they would have until Thursday at midnight to use any "leftover" credits they might still have.
However, another thing to think of: if they only plan on going to a park just one day, how do they plan to use their dining credits? Just using some at their one park day and then the rest at the resort restaurants? Because they would still have to have park tickets for the other days if they have park restaurant reservations - they can't get in the park free to be able to eat at the restaurants, but they can eat at the resorts. Or will they be spending them all up that one day that they do go to a park? (I guess that could be done, if they are really, really big eaters!

)