There is absolutely nothing to be gained by trying to call to reduce your number. At WDW, you are not reserving a specific table. Instead, you are seated after arrival at the next available table for a party of your size. So if only two people show up, you get the next available table for two instead of the next available table for 3. You aren't messing up the restaurant's reservations system by keeping your current number. At a traditional restaurant, they would hold a table for 4, expecting your arrival, and when only two people show up, it shifts things around, as they now need to either have two people occupy a table for 4 and potentially lose revenue, or have you wait for what could be an extended period of time for a two-top to open up. But Disney does not work that way. It's not as if the hostess is going to say: "Gee, Mr. Jones...we had this beautiful table for 4 all set up for you and now you have messed everything up." Rather, the hostess will now say: "We will seat you as soon as a table becomes available" which is exactly what she would have said if you had shown up with three people in your party.
If you call and try to rearrange the deck chairs, odds are that the person on the phone will try to find a table for two, and if they do, it won't be at your current time, which could mess up your schedule. The ADR call center is not in the thick of things and they tend to botch up more than they correct.
