The formula for impulse is F*(t₂-t₁)=m*(v₂-v₁), where F is the force, t is time, m is mass and v is velocity.
In our case m*(v₂-v₁)=16 N s, m=4 kg, v₁=0 and t₁=0.
m*(v₂-0)=16 N s, we solve for v₂:
4*v₂= 16
v₂=16/4
v₂=4 m/s
So the final velocity of the ball is v₂=4 m/s.
