Answer
$$4\left( {\cos 270^\circ + i\sin 270^\circ } \right)$$
Work Step by Step
$$\eqalign{
& {\text{Rectangular Form }} - 4i \cr
& {\text{Use }}r = \sqrt {{a^2} + {b^2}} {\text{ and }}\theta = {\tan ^{ - 1}}\left( {\frac{b}{a}} \right),{\text{ so}} \cr
& r = \sqrt {{{\left( 0 \right)}^2} + {{\left( { - 4} \right)}^2}} = 4 \cr
& \theta = {\tan ^{ - 1}}\left( {\frac{{ - 4}}{0}} \right) \cr
& \theta = 270^\circ \cr
& {\text{write the vector in the trigonometric form }}r\left( {\cos \theta + i\sin \theta } \right) \cr
& = 4\left( {\cos 270^\circ + i\sin 270^\circ } \right) \cr} $$
