4. (5 points) Definition: If $a: b=b: c$, then $b$ is called the mean proportional between $a$ and $c$; for example, $1: 2=2: 4$, so 2 is the mean proportional between 1 and 4. $\qquad$
【Solution】Solution: According to the problem, we have:
$$
\begin{array}{l} 
0.9: 0.6=0.6: x \\
0.9 x=0.6 \times 0.6 \\
0.9 x=0.36 \\
x=0.36 \div 0.9 \\
x=0.4 ; \\
\frac{1}{2}: \frac{1}{5}=\frac{1}{5}: \quad y \\
\frac{1}{2} y=\frac{1}{5} \times \frac{1}{5} \\
y \frac{1}{25} \div \frac{1}{2} \\
y=0.08 \\
x+y=0.4+0.08=0.48 .
\end{array}
$$

Therefore, the answer is: 0.48 .