after you have graphed the function the three "functions" y=x, y=4, x=0. notice you are revolving an infinitisimal vertical strip above y=x and below y=4 the figure would be a cylinder that has taken a cone from it.

for the strip

dV= pi(R2^2-R1^2)dx R2=4 R1=x thus dV =pi(16-x^2)dx integrate V=pi(16x -x^3/3) from 0 to a solving for a y=4, y=x thus x=4 then a=4 V=pi(16x -x^3/3) from 0 to 4 V=((pi(16*4 -(4)^3/3)-0) V=128/3pi u^3 ;-)