Shell method versus disk method

[fiziksfun]

I'm confused when to use the shell method vs. the disk method

Specifically in this problem:

If the region encolsed by the y-axis, the line y=2 and the curve y=\sqrt{x} is revolved around the y-axis, the volume of the solid generated is:

However when I use the shell method and the disk method I get different answers

Can someone tell me what I'm doing wrong?

Disk Method:
\pi \inty^4 dy evaluated from 0 to 2

Shell Method:

2\pi\intx(2-\sqrt{x})dx

Help please!

 

[Vid]

I got the same answer. Remember the second integral is from 0 to 4 not 0 to 2.