4. The great-grandfather has four children, each of his children has four children, and each of those children has four children. How many descendants does the great-grandfather have in total?
4. Since each of the great-grandfather's children has 4 children, the great-grandfather has $4 \cdot 4=16$ grandchildren. 1 POINT As each of the great-grandfather's grandchildren has 4 children, the great-grandfather has $16 \cdot 4=64$ great-grandchildren. 1 POINT Therefore, the number of the great-grandfather's descendants is $4+16+64=84$. $\qquad$