4. (3 points) In the sprint stage of the 400-meter race, A is 30 meters ahead of B, C is 60 meters behind D, and B is 20 meters ahead of C. At this moment, the two leading students are ( ) meters apart.
A. 10
B. 20
C. 50
D. 60
【Analysis】According to the problem, draw the line segment diagram as follows: The two students running in front are Ding and Jia, with a difference of $60-20-30=$ 10 meters.

【Solution】Solution: From the analysis, we get: The two students running in front are Ding and Jia, with a difference of $60-20-30=10$ (meters). Answer: The two students running in front differ by 10 meters; therefore, the correct choice is: A.