To evaluate the expression \( 15x + 4 \) for \( x = -1.5 \), we substitute \(-1.5\) for \(x\):
\[ 15(-1.5) + 4 \]
Calculating \( 15 \times -1.5 \):
\[ 15 \times -1.5 = -22.5 \]
Now, substitute this value back into the expression:
\[ -22.5 + 4 \]
Now perform the addition:
\[ -22.5 + 4 = -18.5 \]
Thus, the value of the expression \( 15x + 4 \) when \( x = -1.5 \) is \(\boxed{-18.5}\).