jmc

First we multiply both sides by $5x-10$ which gives $$x-4=(5x-10)y=5xy-10y$$We can rearrange to $-4+10y=x(5y-1)$. When $5y-1=0$ or $y=\frac{1}{5}$, the left hand side is nonzero while the right hand side is zero, so $\boxed{\frac{1}{5}}$ is unattainable.