4. Compute the following: (a) \( \int_{1}^{2} \frac{1}{3} x^{4} d\left(x^{3}\right) \). Note \( \alpha(x)=x^{3} \). (2 points) (b) \( \int_{2}^{4} x^{2} d\left(e^{x}\right) \). Note \( \alpha(x)=e^{x}