4. The first operation divides the square at the lower left corner of Figure a into four smaller squares, see Figure b; the second operation then divides the smaller square at the lower left corner of Figure b into four even smaller squares, see Figure c; continuing in this manner, after the fifth operation, the resulting figure contains $\qquad$ squares.
Analysis: Find the pattern. Figure a has 5 squares, and with each operation, one square is turned into four smaller squares, increasing the number of squares by 4 each time. Therefore, the answer is $5+5 \times 4=25$.