4. (2002 Hunan Province Competition Question) The number of sets $X$ that satisfy the condition $\{1,2,3\} \subseteq X \subseteq\{1,2,3,4,5,6\}$ is $\qquad$
4. Fill 8. Reason: $X$ must include the 3 elements $1, 2, 3$, while the numbers 4, 5, 6 can either belong to $X$ or not belong to $X$, with each number having 2 possibilities. Therefore, the total number of different $X$ is $2^{3}=8$.