\[\frac{x - 4\sqrt{\text{xy}} + 4y}{x - 4y} =\]
\[= \frac{\left( \sqrt{x} - 2\sqrt{y} \right)^{2}}{\left( \sqrt{x} - 2\sqrt{y} \right)\left( \sqrt{x} + 2\sqrt{y} \right)} =\]
\[= \frac{x - 2\sqrt{y}}{x + 2\sqrt{y}}.\]