To find the value of the function \( y = \frac{1}{8}x - 13 \) when \( x = -4 \), we can substitute \(-4\) into the equation:
\[ y = \frac{1}{8}(-4) - 13 \]
Now, calculate \(\frac{1}{8}(-4)\):
\[ \frac{1}{8}(-4) = -\frac{4}{8} = -\frac{1}{2} \]
Now substitute this back into the equation:
\[ y = -\frac{1}{2} - 13 \]
To combine these, convert \(-13\) to a fraction with the same denominator:
\[ -13 = -\frac{26}{2} \]
Now add the fractions:
\[ y = -\frac{1}{2} - \frac{26}{2} = -\frac{27}{2} \]
Convert \(-\frac{27}{2}\) to a decimal for easier comparison:
\[ -\frac{27}{2} = -13.5 \]
So the value of the function when \( x = -4 \) is:
\[ \boxed{-13.5} \]