Volume using Shell method. Stuck on last step!

The volume of this solid can also be computed using cylindrical shells via an integral

with limits of integration http://math.webwork.rochester.edu:80...19ca0ec061.png 0 and http://math.webwork.rochester.edu:80...d84f51ed91.png 4

volume is http://math.webwork.rochester.edu:80...f7e0799941.png cubic units.

I am stuck on the last step. For some reason I tried integrating it and then plugging in the value, which in this case is just the beta, 4.

What am I doing wrong? this is supposed to be easy step..