WELCOME TO THE SQUARE ROOTS OF FOUR!

HERE'S HOW YOU FIND THEM:

(4+0i) has length or magnitude 4 and an angle to the x-axis, or modulus, of zero degrees;
so the root circle has radius 4^1/2 = 2,
the first root is at theta1 = 0/2 = 0 degrees,
and the remaining root is at theta = 0 + 360/2 = 180 degrees.