Volume of tank=πr2h where r=1.5m, r2=2.25m2, h=9m. Volume=20.25πm3 (about 63.6173m3).
Velocity of flow = 4m/s has no meaning because we don't know how the water is drained from the tank (e.g. by a cylindrical pipe of a given diameter) .
Assuming rate of flow=4m3/s, then the tank will take 63.6173/4=15.9 seconds approx to empty.
Alternatively, if the tank is actually a large horizontal pipe and water is moving at a rate of 4m/s, then the length of the pipe is irrelevant and the volume of water moving at the given velocity gives us a cylindrical volume (h=4m) of 9πm3/s (28.274m3/s approx) as the rate of flow of the water along the pipe.
1cc≡1mL=0.001L; 1cm=0.01m, 1cc=(0.01)3m3=0.001L, so 1m3=0.001/(0.01)3L=1000L.
The discharge rate is therefore about 28,274L/s.