It is much easier to go down from a 6 top to a 4 top; most restaurants are series of 2 tops.
If you went in and said that you were going from a party of 4 to a party of 6, they would most likely not be able to accommodate you right away or at all.
If you are going from a party of 6 to a party of 4, and the restaurant is very busy, the hostess will probably hug you!
You can call the dining reservations line and go ahead and bump down the parties to 4 if you would like to assist the restaurant ahead of time in setting up their seating for that time period. It will not effect your ressie at all either way.