A chord across a circle forms an isosceles triangle by joining the centre of the circle
to the two endpoints of the chord.
An isosceles triangle in turn is formed of two back-to-back identical right-angled triangles.
Now you may use Pythagoras' theorem, since half of the chord is 4 units long.
That gives one of the perpendicular sides of one of the identical right-angled triangles.
The hypotenuse is the radius.
All that remains is to calculate the other perpendicular side using Pythagoras' theorem.