For this case what you should do is use the following trigonometric relationship:
sin (x) = C.O / h
Where
x: angle
C.O: opposite leg
h: hypotenuse
Substituting the values we have:
sen (60) = CD / BC
sen (60) = CD / 4
CD = 4 * sin (60)
CD = 3.46
Answer:
CD = 3.46
