4. (10 points) 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 $c$; this process continues, and after the sixth operation, the resulting figure contains $\qquad$ squares.
【Analysis】Figure $a$ has 5 squares, and each subsequent operation transforms one square into four smaller squares, increasing the number of squares by 4 each time. Therefore, the answer is $5+6 \times 4=29$.

【Solution】Solution: $5+6 \times 4=29$.
So the answer is: 29.