4. The perimeter of a cardboard sheet in the shape of a square is 28 dm. How many square centimeters does its area contain?
4. 5) $28: 4=7$ (dm) - the length of the side of the square;
2) $7 \times 7=49$ (sq. dm) - the area of the square;
3) $100 \times 49=4900$ (sq. cm).

Answer: the area of the square is 4900 sq. cm.