The center of the circle is the midpoint of any diameter.
So, center = ((-4+4)/2, (-5+1)/2) = (0, -2)
Radius = half the length of the diameter = (1/2)√ [(-4-4)2 + (-5-1)2] = 5
Equation of circle has the form (x-h)2 + (y-k)2 = r2, where (h,k) is the center and r is the radius.
So, we have (x-0)2 + (y-(-2))2 = 52
Simplify to get x2 + (y+2)2 = 25