The velocity is changing linearly (constant acceleration) so the average velocity over any time interval can be calculated as either of:
.... the average of the starting and ending velocities, or
.... the velocity at the midpoint (in *time*, not distance!).
So the average velocity over [4 s, 5 s] is the speed at t=4.5 s:
v = 67 km/h - (0.45 m/s^2)(4.5 s)
= 67 km/h * [1000 m / 1 km] * [1 h / 3600 s] + 2.025 m/s
= 18.61 m/s - 2.03 m/s
= 20.64 m/s
That's too many digits, but I like to round at the very end. In one second at that average speed you travel about 20.64 meters, rounding to 21.6 assuming the 67 km/h initial speed was within about 0.5 m/s which is about 0.14 m/s; which I think is small enough to quote the first decimal place. The delta-v figure has 2 significant digits, so 2.03 m/s is good to the first decimal place, too.