4. Simplify the expression \( \frac{16ab}{-36a^2b} \).

Divide numerator and denominator by their common factors: \( \frac{16}{-36} = \frac{-4}{9} \), \( a \) reduces to \( a^{-1} \), and \( b \) cancels out. The result is \( \frac{-4}{9a} \).