4. The equation of the tangent line to the curve $y=5 \sqrt{x}$ that is parallel to the line $y=2 x-4$ is
4. $16 x-8 y+25=0$. Hint: $\left.y^{\prime}\right|_{x=x_{0}}=\frac{5}{2 \sqrt{x_{0}}}$, thus $x_{0}=\frac{25}{16}$. $y_{0}=\frac{25}{4}$.