4. (10 points) A box contains several black and white chess pieces. If one black piece is taken out, the ratio of the remaining black pieces to white pieces is $9: 7$. If the black piece is put back and then one white piece is taken out, the ratio of the remaining black pieces to white pieces is $7: 5$. Therefore, the number of black pieces originally in the box is more than the number of white pieces by ( ) pieces.
A. 5
B. 6
C. 7
D. 8
【Analysis】We use proportions to solve the problem. Let the number of white stones be $x$, and the number of black stones be $\frac{9}{7} x+1$. The ratio of the number of black stones to the number of white stones minus 1 is 7:5. We can set up an equation to solve it.

【Solution】Let the number of white stones be $x$, and the number of black stones be $\frac{9}{7} x+1$.
$$
\begin{aligned}
\left(\frac{9}{7} x+1\right):(x-1) & =7: 5, \\
\frac{9}{7} x \times 5+5 & =7 x-7, \\
6 \frac{3}{7} x+5 & =7 x-7, \\
\frac{4}{7} x & =12, \\
\frac{4}{7} x \times \frac{7}{4} & =12 \times \frac{7}{4}, \\
x & =21 ;
\end{aligned}
$$

The number of black stones:
$$
\begin{array}{l}
\frac{9}{7} x=\frac{9}{7} \times 21+1=28 ; \\
28-21=7 \text { (pieces); }
\end{array}
$$

Therefore, the answer is: $C$.