To solve for \( y \) in the equation \( 3x + y = 9 \) given that \( x = 4 \), we can substitute \( x \) into the equation:
\[ 3(4) + y = 9 \]
Calculating \( 3(4) \):
\[ 12 + y = 9 \]
Next, we isolate \( y \) by subtracting 12 from both sides:
\[ y = 9 - 12 \] \[ y = -3 \]
Therefore, the solution is \( (x, y) = (4, -3) \).
So, the final answer is \( (4, -3) \).