To evaluate the expression \(15x + 4\) for \(x = 1.5\), we substitute \(1.5\) for \(x\):
\[ 15(1.5) + 4 \]
Now, calculate \(15 \times 1.5\):
\[ 15 \times 1.5 = 22.5 \]
Next, add \(4\) to \(22.5\):
\[ 22.5 + 4 = 26.5 \]
Therefore, the value of the expression \(15x + 4\) when \(x = 1.5\) is:
\[ \boxed{26.5} \]