a)  Make a sketch.

    One single small circle has the radius of 4 cm.
    Its center is located at the distance 12-4 = 8 cm from the center O.
    You easily get the right angled triangle with the hypotenuse of 8 cm and the leg of 4 cm.
    Its acute angle is 30 degs; so one small circle blocks the central angle of 60 degs.
    And EACH such circle blocks 60 degs.
    So, 4 circles block 240 degs.


b)  If 4 interior circles block the entire circumference, it implies that each small circle blocks the central angle of 90 degs.

    Then it is easy to see that the distance from O to the center of each small circle is  cm.