4. Place the numbers $1,2,3,4,5,6,7,8$ on the eight vertices of a cube, such that the sum of any three numbers on each face is not less than 10. Then the minimum value of the sum of the four numbers on each face is $\qquad$ .
4. 16. Let the sum of four numbers $a_{1}, a_{2}, a_{3}, a_{4}$ on a certain face reach the minimum value, and $a_{1}6$, so $a_{1}+a_{2}+$ $a_{3}+a_{4} \geqslant 16$. The figure on the right illustrates that 16 can be achieved.