Complete the square.
[tex]z^4 + z^2 - i\sqrt 3 = \left(z^2 + \dfrac12\right)^2 - \dfrac14 - i\sqrt3 = 0[/tex]
[tex]\left(z^2 + \dfrac12\right)^2 = \dfrac{1 + 4\sqrt3\,i}4[/tex]
Use de Moivre's theorem to compute the square roots of the right side.
[tex]w = \dfrac{1 + 4\sqrt3\,i}4 = \dfrac74 \exp\left(i \tan^{-1}(4\sqrt3)\right)[/tex]
[tex]\implies w^{1/2} = \pm \dfrac{\sqrt7}2 \exp\left(\dfrac i2 \tan^{-1}(4\sqrt3)\right) = \pm \dfrac{2+\sqrt3\,i}2[/tex]
Now, taking square roots on both sides, we have
[tex]z^2 + \dfrac12 = \pm w^{1/2}[/tex]
[tex]z^2 = \dfrac{1+\sqrt3\,i}2 \text{ or } z^2 = -\dfrac{3+\sqrt3\,i}2[/tex]
Use de Moivre's theorem again to take square roots on both sides.
[tex]w_1 = \dfrac{1+\sqrt3\,i}2 = \exp\left(i\dfrac\pi3\right)[/tex]
[tex]\implies z = {w_1}^{1/2} = \pm \exp\left(i\dfrac\pi6\right) = \boxed{\pm \dfrac{\sqrt3 + i}2}[/tex]
[tex]w_2 = -\dfrac{3+\sqrt3\,i}2 = \sqrt3 \, \exp\left(-i \dfrac{5\pi}6\right)[/tex]
[tex]\implies z = {w_2}^{1/2} = \boxed{\pm \sqrt[4]{3} \, \exp\left(-i\dfrac{5\pi}{12}\right)}[/tex]
