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.