The volumes for each part are as follows:
- a. Volume around the \( x \)-axis: \( 8\pi \)
- b. Volume around the \( y \)-axis: \( \frac{16\pi}{5} \)
- c. Volume around the line \( y = 4 \): \( \frac{40\pi}{3} \)
Thus, the final answers are:
\[
\boxed{8\pi}, \quad \boxed{\frac{16\pi}{5}}, \quad \boxed{\frac{40\pi}{3}}
\]