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 fifth 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 total number of squares by 4 each time. Therefore, the formula can be derived as: $5+n(n-1)$.

【Solution】Solution: When the fifth operation is completed, the total number of squares in the resulting figure is:
$$
\begin{array}{l}
5+4 n \\
=5+4 \times 5 \\
=25 \text { (squares) }
\end{array}
$$

Answer: When the fifth operation is completed, the resulting figure contains a total of 25 squares.
Therefore, the answer is: 25