It isn't wrong, but it is a slightly different sentence.
In "Kvar kvaronoj estas unu", you are emphasizing an equality...4 * 1/4 = 1. "Kvar kvaronoj faras unu", on the other hand, emphasizes the putting together.
Now, when translating, I can't see why Duolingo shouldn't accept both. There are other sentences of this type that use "estas".
please add some arguments why you know or think so. Examples are great, too. How would you then translate ''... 3 1/3...'' if not with ''..three thirds...'' if I may ask ? Do you see the point ? With a single example I found an argment against your opinion, right ;-) No harm, though.
just mina deux cents...