To find the value of the function \( y = 3x^2 - 21 \) when \( x = 4 \), we can substitute \( 4 \) for \( x \) in the equation:
\[ y = 3(4)^2 - 21 \]
Calculating \( (4)^2 \):
\[ (4)^2 = 16 \]
Now substitute that back into the equation:
\[ y = 3(16) - 21 \]
Now calculate \( 3(16) \):
\[ 3(16) = 48 \]
So now we have:
\[ y = 48 - 21 \]
Finally, subtract \( 21 \) from \( 48 \):
\[ y = 27 \]
Therefore, the value of the function \( y = 3x^2 - 21 \) when \( x = 4 \) is \( \boxed{27} \).