Try this

Let 小红go from A to B.
At A 小红see 2 cars from both ways, and when arrive at B小红see 2 cars from both ways.

The key is to understand 10 cars 发车from each side, and 发车时间间隔is the time for a car runs from A to B.

Suppose two observers a at A, and b at B. Both a and b will see cars from both side with时间间隔 as 发车时间间隔. Both of them see 10 cars.

(It will be clearer if we think from the view of a. 迎面来的公共汽车11th辆is still at B, which 小红 will see. And 后面追过来的公共汽车10th辆 is at A, which 小红 will not see. So 小红到家时，迎面来的公共汽车数了11辆，后面追过来的公共汽车数了9辆.)

So it takes 10 发车时间间隔s for小红go from A at B. or 发车时间间隔is 1/10 of time for 小红from A to B.

When the 迎面发车9th car is at A, and the 迎面发车10th car is at B. So it takes the 9th car 1 发车时间间隔 to run from A to B. So it takes 1/10 of the time for 小红.

v=10x4千米每时=40千米每时

Futher questions will be:
到家时，迎面来的公共汽车数了21辆，后面追过来的公共汽车数了17辆。

到家时，迎面来的公共汽车数了31辆，后面追过来的公共汽车数了25辆。

The answer for the 2 questions should be 40千米每时.