Is it permissible for a single sacrifice to be offered on behalf of two deceased grandparents or four deceased individuals?

There is no harm in offering a sacrifice on behalf of any number of deceased individuals or in including them in the reward of your sacrifice. However, if you were to donate its value on their behalf, it would be more preferable, because some scholars, such as the Shafi'is, believe that it is not permissible to offer a sacrifice on behalf of the deceased unless they have willed it.