First of all, observe that
[tex]\sin(240)=\sin(-30+270)=-\cos(-30)=-\cos(30)=-\dfrac{\sqrt{3}}{2}[/tex]
And similarly,
[tex]\cos(240)=\cos(-30+270)=\sin(-30)=-\sin(30)=-\dfrac{1}{2}[/tex]
Now just substitute these values:
[tex]4(\cos(240)+i\sin(240)=4\left(-\dfrac{1}{2}+i\dfrac{-\sqrt{3}}{2}\right) = -2-2i\sqrt{3}[/tex]
And since the complex number a+bi is mapped to the point (a,b), the rectangular coordinates are
[tex](-2, -2\sqrt{3})[/tex]
