#### Answer

The velocity at t = 4 seconds is $4.5 ~m/s$.

#### Work Step by Step

From the graph, we can see that the average force from 0 to 3 seconds is 3.0 N. We can use the average force to find the average acceleration from 0 to 3 seconds;
$F_{ave} = m~a_{ave}$
$a_{ave} = \frac{F_{ave}}{m}$
$a_{ave} = \frac{3.0~N}{2.0~kg}$
$a_{ave} = 1.5~m/s^2$
We can find the velocity at t = 3 seconds;
$v_f = v_0+a_{ave}~t$
$v_f = 0+(1.5~m/s^2)(3~s)$
$v_f = 4.5~m/s$
Since the force from t = 3 s to t = 4 s is zero, the acceleration is also zero. Therefore, the velocity at t = 4 s is equal to the velocity at t = 3 s.
The velocity at t = 4 seconds is $4.5 ~m/s$.