4. All the triangles in the figure below are equilateral triangles. The side length of the red triangle is 24.7, and the side length of the blue triangle is 26. What is the side length of the green triangle?
4. Solution:

In the figure, there are 15 small triangles, for convenience, we have given them numbers. Among these small triangles, there are 5 pairs with equal side lengths, which are 4 and 5, 7 and 8, 9 and 10, 11 and 12, 14 and 15 (filled with the same color respectively). Extending the left side of 6 (indicated by a thin red line in the figure), it can be seen that the difference in side lengths between 13 and 14 is equal to the difference in side lengths between 1 and 2, which is $26 - 24.7 = 1.3$.

Let the side length of 14 and 15 be $\mathrm{a}$, and use $b_{x}$ to represent the side lengths of each triangle, then $b_{15}=b_{14}=\mathrm{a}, b_{13}=\mathrm{a}$
$$
\begin{array}{l}
+1.3, b_{11}=2 \mathrm{a}+1.3, b_{9}=b_{10}=3 \mathrm{a}+1.3, b_{7}=3 \mathrm{a}+2.6, b_{6}=4 \mathrm{a}+1.3, \\
b_{4}=4 \mathrm{a}+3.9=5 \mathrm{a}+1.3, \\
\therefore \mathrm{a}=2.6, b_{10}=9.1 \\
\text { Thus } b_{3}=b_{2}-b_{10}=24.7-9.1=15.6
\end{array}
$$