4 1/2 is equal to:
9/2
18/4
36/8
You get the first one by taking the 4, multiplying it by the denominator, in this case 2, and adding that to the top. So you end up with 9/2. Then you can double that, what you get, in this case 18/4, is equal to the one before that. You could keep doubling it forever and ever and ever and it would still be equal to 4 1/2.
Hope this helps!