\[y = \sqrt{4x - x^{2}}\]
\[4x - x^{2} \geq 0\]
\[4x - x^{2} = 0\]
\[x(4 - x) = 0\]
\[x = 0,\ \ x = 4\]
\[D(y) = \lbrack 0;4\rbrack.\]