Finding vector magnitude

Find a vector whose magnitude is 4 and whose x component is twice its y component.

I know that to calculate magnitude it is r= sqrt (x1 + x2)2 + (y1 + y2)2.

I also know that k(vector)= 4

I just am confused by the components (so x=2y ....)

so that $4=\sqrt{x^2+y^2}$...(*)