4. (5 points) There are two sets of numbers, the average of the first set is 15, and the average of the second set is 21. If the average of all numbers in these two sets is 20, then, the ratio of the number of elements in the first set to the number of elements in the second set is $\qquad$ .
【Solution】Solution: Treat the total number as "1", let the first group be $x$, then:
$$
\begin{aligned}
15 x+21 \times(1-x) & =20 \times 1 \\
15 x+21-21 x & =20 \\
6 x & =1 \\
x & =\frac{1}{6}
\end{aligned}
$$

Then the second group is: $1-\frac{1}{6}=\frac{5}{6}$
Their ratio is: $\frac{1}{6}: \frac{5}{6}=1: 5$.
Therefore, the answer is: $1: 5$.