Solution: (1) According to the meaning of the question, there are 20 red balls and 30 yellow balls in 50 ball-touching experiments.

The percentage of red balls is 2050 = 40%;

The percentage of yellow balls is 30 ÷ 50 = 60%;

A: Red balls account for 40% and yellow balls account for 60%.

(2) According to the meaning of the question, in 50 ball touching experiments, the marked ball appeared four times.

The total number of balls is 50÷4x8= 100.

The number of red balls is100x40 = 40 40.

There are 40 red balls in the box.