-4i is a complex number, and complex number can be seen as vector too.
Polar form of a number, on the other hand is representation of the number in polar coordinates. So, converting complex number into its polar representation is writing the complex number in terms of a distance from the origin and a direction (or angle) from the positive horizontal axis.
The polar form of complex number can be written as:
x+yj=r(cos(alfa)+sin(alfa)), where r is the absolute value (the modulus) of the complex number and alfa is the argument of the complex number.
r=sqrt(x^2+y^2)=sqrt(4^2+0)=sqrt(16)=4
alfa=tanges^(-1)(0/4)=tanges(0)=0
-4i=4*(1+0)=4
