Depends on where you want to go....they will have to see if another reservation for your party size is available at that time and book that before cancelling your existing one. 4 people is a lot to add, they sometimes have a hard time adding even one person to a reservation (at places like Le Cellier and the hard to get ones). If the restaurant is booked solid, you won't be able to get them in....go to the dining reservations online and see what you can get, I'd do it ASAP if I were you!
We're going in December and have 9 people in our party, and it was difficult to get some of the places we wanted even 180 days out!