To find the value of the function \( y = \frac{1}{8}x - 13 \) when \( x = -4 \), we substitute \(-4\) into the equation.
\[
y = \frac{1}{8}(-4) - 13
\]
First, calculate \( \frac{1}{8} \times (-4) \):
\[
y = -\frac{4}{8} - 13
\]
Simplifying \(-\frac{4}{8}\):
\[
-\frac{4}{8} = -\frac{1}{2}
\]
Now, substituting this back in:
\[
y = -\frac{1}{2} - 13
\]
Next, we need to express \( -13 \) in terms of halves to combine the fractions:
\[
-13 = -\frac{26}{2}
\]
So we can rewrite the equation:
\[
y = -\frac{1}{2} - \frac{26}{2}
\]
Now, combine the fractions:
\[
y = -\frac{1 + 26}{2} = -\frac{27}{2}
\]
Thus, the value of the function when \( x = -4 \) is:
\[
y = -\frac{27}{2} \text{ or } -13.5
\]
So the final answer is:
\[
\boxed{-\frac{27}{2}} \text{ or } -13.5
\]