4. (20 points) For two perpendicular lines $a$ and $b$ on a plane, $(a, b)$ is called a "perpendicular pair", and both $a$ and $b$ belong to this "perpendicular pair". How many "perpendicular pairs" can be formed at most when there are twenty lines on the plane?
【Analysis】As shown in the figure: When twenty lines have 10 lines parallel to each other; the other 10 lines are not only parallel to each other but also perpendicular to the first 10 lines, the maximum number of perpendicular pairs is 100.

【Solution】Solution: When twenty lines have 10 lines parallel to each other; the other 10 lines are not only parallel to each other but also perpendicular to the first 10 lines, the maximum number of perpendicular pairs is:
$$
10 \times 10=100 \text { (pairs) }
$$

Answer: When there are twenty lines on a plane, the maximum number of "perpendicular pairs" that can be formed is 100.