Depends on the restaurant. Some places like Garden Grille at Epcot might say they'll have to split the group. But in most cases, they should be able to put tables together to accommodate 8.
I think the PS system has a problem only with odd-numbered groups. When I tried to do 3, the CM said the system kinda "choked", so we did 4 and when we arrived told them 1 was not coming.