To evaluate the expression
\[ 9 - \dfrac{8}{S} \]
when \( S = 4 \), we first substitute the value of \( S \) into the expression:
\[ 9 - \dfrac{8}{4} \]
Next, we calculate \( \dfrac{8}{4} \):
\[ \dfrac{8}{4} = 2 \]
Now we substitute this value back into the expression:
\[ 9 - 2 \]
Finally, we perform the subtraction:
\[ 9 - 2 = 7 \]
Thus, the evaluated result is
\[ \boxed{7} \]