The mean and variance of the data \( 4, 5, 6, 6, 7, 8, x, y \), where \( x < y \), are \( 6 \) and \( \frac{9}{4} \) respectively. Then \( x^4 + y^2 \) is equal to
\( 162 \)
\( 320 \)
\( 674 \)
\( 420 \)