4. For an equilateral triangle $ABC$, divide each side into $n$ equal parts, and draw lines parallel to the sides through each division point, forming a grid of equilateral triangles. Try to find the number of rhombi with side length $\frac{1}{n}$ in this grid.
4. Referring to Example 11, the number of small rhombi is $3 C_{n}^{2}$.