The formula that you must used to determine the circumference of a circle is C = 
 d
d
First things first, substitute your diameter into the formula provided above, 
C = 
 (3.14)(4)
(3.14)(4)
Next you've substituted the diameter into the formula, you may calculate it.
C = 
 (3.14)(4) = 12.57
(3.14)(4) = 12.57
Finally, it has been determined that the answer to your question is 12.57