4. Divide a regular hexagon into 6 congruent equilateral triangle regions $A, B, C, D, E, F$, and plant ornamental plants in these 6 regions. The requirement is that the same type of plant is planted in the same region, and different types of plants are planted in adjacent regions. There are 4 different types of plants available, so there are
ways to plant the plants.
4. 732. When considering planting the same type of plant at $A$, $C$, and $E$, there are $C_{1}^{1} \times C_{3}^{1} \times C_{3}^{1} \times C_{3}^{1}=108$ methods. When considering planting 2 different types of plants at $A$, $C$, and $E$, there are $C_{4}^{2} C_{3}^{2} C_{2}^{1} C_{3}^{1} C_{2}^{1} C_{2}^{1}=432$ methods. When considering planting 3 different types of plants at $A$, $C$, and $E$, there are $A_{4}^{3} C_{2}^{1} C_{2}^{1} C_{2}^{1}=192$ methods. In total, there are $108+432+192=732$ methods. Alternatively, using the formula from Example 16 in the recursive method section with $(m=4, n=6)$, we get a total of $(4-$ $1)^{6}+(-1)^{6}(4-1)=732$ methods.