Answer
$Y_{min}=1,Y_{min}=9,Y_{scl}=1$
Work Step by Step
We know that for a rectangle to result in a square screen, $\frac{X_{max}-X_{min}}{Y_{max}-Y_{min}}=\frac{3}{2}$ must be true.
Here: $\frac{X_{max}-X_{min}}{Y_{max}-Y_{min}}=\frac{8-(-4)}{Y_{max}-Y_{min}}=\frac{12}{Y_{max}-Y_{min}}=\frac{3}{2}$. Hence $Y_{max}-Y_{min}=8$. Also, the point $(4,8)$ must be in the viewing rectangle.
Hence if e.g. $Y_{min}=1,Y_{min}=9,Y_{scl}=1$, all conditions are satisfied.