To find the value of the expression \( 10w - 4 \) when \( w = 8 \), substitute \( 8 \) for \( w \):
\[ 10w - 4 = 10(8) - 4 \]
Now, calculate \( 10 \times 8 \):
\[ 10 \times 8 = 80 \]
Next, subtract \( 4 \):
\[ 80 - 4 = 76 \]
Therefore, the value of the expression \( 10w - 4 \) when \( w = 8 \) is \( \boxed{76} \).