Simple! In order to find the answer you’d take your current ratio (1:4) and you’d find the equivalent of that when your ratio is (x:2). So you’d set up a fraction and cross multiply.
[1/4 = x/2] → [2 = 4x]
Then you solve for x by dividing both sides by 4.
[2 = 4x] → x = 2/4 or x = .5
The answer is .5 centimeters.