This answer is related to changing lanes. The importance of the four seconds, is the necessary amount of time for the person in front of you and the person behind you to brake properly in order to avoid a collision, should something happen on the road. A four second gap, means that as you pull into the middle of the two vehicles as you are changing lanes, the car in front should be ahead by two second and the person behind you should be two seconds behind, a total of four seconds.