Solutions; (1) Draw the following polyline statistical chart:

② As can be seen from the statistical chart of dotted line, the scores of unit 3 and unit 4 dropped the fastest;

③ The highest score is the second unit;

As can be seen from the picture, Tao Tao's performance began to decline from Unit 3, and it declined rapidly.

So the answer is: 2 3, 4, 3 2.