Some hints:

• 4.17: What is $i$ in polar form? Then remember that taking the square root is the same as raising to the $1/2$ power (and De Moivre's formula still works for fractional powers.)
• 4.26: How did you solve 4.25? Both can actually be done using a similar method. Like 4.17 start by converting 4i into polar form. Note to get all the roots, remember adding $2\pi$ or $360^\circ$ to the argument gives the same complex number.
• 4.28: $z$ has modulus $1$, so it can be written in polar form as $\cos(\theta) + i\cdot \sin(\theta)$ for some $\theta$. Try dividing the numerator and denominator of the expression by $z^n$ so the denominator is $z^{-n} + z^{n}$ and calculate from there.

Hope these hints help!