If the car is accelerating uniformly then,
velocity of car at the fourth second is 16m/s
Then if the car with this velocity stopped after 2s then acceleration is,
      a=0-16/2=-8m/s^2
braking distance is given by
 d=16*2-0.5*8*2*2
   =32-16
   =16m
this answer is valid only if the motion is one dimensional
all the best.