We need more information.
A full description of the figure is required.
Presently, I am guessing the following "facts" . . .
We have a quadrant of a circle with radius 4 (the "4-circle").
There is a semicircle of radius 2 (the "2-circle").
The "4 cm" at the bottom is not the length of the chord.
. . It is obviously a radius of the 4-circle.
We have a mystery circle tangent to the 2-circle.
It appears to extend beyond the 4-circle.
With no further constraints, it could have any radius.