4. (10 points) The diagram below shows the street layout from Lulan's home to her school. Lulan has $\qquad$ different shortest routes from home to school.
18 different shortest routes,
【Analysis】According to the problem, the number of ways can be marked using the labeling method. To find the shortest route, Lanlan must pass through the middle diagonal line. Therefore, we only need to calculate the number of ways before and after crossing the diagonal line to reach the school.

【Solution】According to the analysis, using the labeling method (the number at each grid point is the sum of the numbers from the adjacent left and bottom points), 

from $A$ to $C$ there are 3 different routes, and from $D$ to $B$ there are 6 different routes. Therefore, the total number of routes from Lanlan's home to the school is $3 \times 6=$ hence the answer is: 18.