4. Given a point $M$ in the first quadrant on the graph of the inverse proportion function $y=\frac{5}{x}$, draw $M A \perp x$-axis at point $A$, and draw $M B \perp y$-axis at point $B$. If $M A$ and $M B$ intersect the graph of the inverse proportion function $y=\frac{3}{x}$ at points $C$ and $D$ respectively, then $C D: A B$ equals ( ).
(A) $2: 5$
(B) $3: 5$
(C) $1: 3$
(D) $2: 3$
4. A.

Let's assume point $M(1,5)$. Then
$$
\begin{array}{l}
A(1,0), B(0,5), C(1,3), D\left(\frac{3}{5}, 5\right) . \\
\text { Hence } C D=\sqrt{(5-3)^{2}+\left(1-\frac{3}{5}\right)^{2}}=\frac{\sqrt{104}}{5}, \\
A B=\sqrt{1^{2}+5^{2}}=\sqrt{26} . \\
\text { Therefore, } \frac{C D}{A B}=\frac{\sqrt{104}}{5} \times \frac{1}{\sqrt{26}}=\frac{2}{5} .
\end{array}
$$