You should already know
[tex]\cos 30^\circ = \dfrac{\sqrt{3}}{2}[/tex]
[tex]\sin 30^\circ = \dfrac 1 2[/tex]
Trig as taught really mostly uses 30/60/90 triangles and 45/45/90 triangles. Learn how these are derived and memorize them and you'll be well on your way.
But to do the problem,
[tex]\sin 30^\circ = \dfrac{\textrm{opp}}{\textrm{hyp}} = \dfrac{4}{8} = \dfrac 1 2[/tex]
[tex]\cos 30^\circ = \dfrac{\textrm{adj}}{\textrm{hyp}} = \dfrac{4\sqrt{3}}{8} = \dfrac{\sqrt{3}}{2}[/tex]
