In the given question the information's given are enough to find the answer. The first thing that is required to be done is writing down all the information's that are provided in the question.
Speed of the car = 20 m/s
Time taken to reach a complete stop = 4 seconds.
Then
Speed = Distance traveled/Time taken
Distance traveled = Speed * time taken
= (20 * 4) meters
= 80 meters
So the car will stop after traveling a distance of 80 meters.