There is a chance that the extra 2 will be turned away. It all depends on the restaurant, how busy or popular it is, what time the res is for and if you're going during a busy time of year. Castmembers will do their best to seat your 2 extra people, but there's no guarantee that it will be possible because restaurants have to abide by the fire code regulations so if the restaurant is full, anyone not on a reservation can be turned away. Also, we were at a restaurant that was running far behind schedule so it was also turning away walk ups, latecomers and anyone not on a reservation.
As much as possible keep checking online multiple times a day for a reservation for 6 or for 2 at very close to your current res. If you get a table for 2 you can ask to be seated together, but it is only a request and a certainty. Keep checking even after you arrive at DW. You can also subscribe to the cancellation thread on this Dining Reservations board. With the cancellation/no show penalty people will cancel when their plans change.