From the given information, we don't know. All we can tell is the net effect of all the changes during those 4 seconds ... the "averages".
The average change in speed is an increase of 72 km/hr (20 m/s) during that time.
The average acceleration is a constant 18 km/hr/sec (5 m/s^2) during the same time.
But both the speed and the acceleration may have gone up or down many times during the 4 seconds.
