4. (8 points) Two identical equilateral triangles can form a rhombus. If the side length of the equilateral triangle is 10, then the area of the largest square that can fit inside this rhombus is $\qquad$ .
【Analysis】To maximize the area of the square, it only happens when the diagonal of the square aligns with one of the diagonals of the rhombus, allowing the side length of the square to reach its maximum value. Then, the side length of the square can be calculated, followed by its area.

【Solution】There are two methods to place the square inside the rhombus as shown in Figure A and Figure B: According to the analysis, when the diagonal of the square aligns with one of the diagonals of the rhombus, i.e., as shown in Figure A, the side length of the square can reach its maximum value.

When the square is placed as shown, the area is maximized, and the maximum area is: $10 \times 10 \div 2=50$. Therefore, the answer is: 50.