Answer
$$16\sqrt 2 \ \pi.$$
Work Step by Step
Since $y=x$ then $y'=1$. Hence the surface area is given by
$$2\pi\int_0^4 y\sqrt{1+(y')^2}dx =2\pi\int_0^4 x\sqrt{1+1}dx\\
=2\sqrt 2\pi\int_0^4 x dx=\sqrt 2 \pi x^2|_0^4=16\sqrt 2 \ \pi.$$
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