4. If Xiaozhang's daily sleep time is uniformly distributed between 6 hours and 9 hours, then the probability that Xiaozhang's average sleep time over two consecutive days is no less than 7 hours is $\qquad$ .
Answer
As shown in the figure, let the sleep times of Xiao Zhang for two consecutive days be $x, y$, then the distribution area of $(x, y)$ is a square $A B C D$. Draw the line $x+y=14$, the region above it satisfies the average sleep time is no less than 7 hours, i.e., the shaded area in the figure. Since $K(6,8), L(8,6) \Rightarrow S_{\triangle B K L}=2$,
Let $A=$ “Xiao Zhang's average sleep time over two consecutive days is no less than 7 hours”,
Therefore, $P(A)=1-\frac{2}{9}=\frac{7}{9}$.