To find the value of the function \( y = \frac{1}{8}x - 13 \) when \( x = -4 \), we can substitute \(-4\) for \(x\) in the equation:
\[ y = \frac{1}{8}(-4) - 13 \]
Calculating \(\frac{1}{8}(-4)\):
\[ \frac{1}{8}(-4) = -\frac{4}{8} = -\frac{1}{2} \]
Now, substituting this back into the equation:
\[ y = -\frac{1}{2} - 13 \]
To combine the terms, we convert \(-13\) to a fraction that has a denominator of \(2\):
\[ -13 = -\frac{26}{2} \]
Now we can add:
\[ y = -\frac{1}{2} - \frac{26}{2} = -\frac{1 + 26}{2} = -\frac{27}{2} \]
This is equivalent to:
\[ y = -13.5 \]
Therefore, the value of the function \( y \) when \( x = -4 \) is:
\[ \boxed{-13.5} \]