I know you can say things like "a 1/4 inch hole" and similar, where the article belongs to the noun and not to the quarter. A colleague of mine is about to publish something like "a 1/4 of the people voted for...". I'm not a native speaker, but my feeling is that an article is built into "1/4", and that you could simply write "1/4 of the people voted for...". Am I correct? I googled and searched on here but could not find any rules about this.
Also supporting my approach is that if you read "1/4" out as "one fourth" rather than "[a] quarter", you'd get "a one fourth" when including the article which obviously doesn't sound good.
On the other hand, if you need a definite article, you'd have to say "The 1/4 that voted for...", which might support the inclusion of the indefinite article in the other case.
EDIT: I should have mentioned that using "1/4" in this case was for stylistic reasons, for use on an infographic, but I would think that some grammar rules would still have to be observed.
EDIT 2: I looked up the actual example now, and it was "In a 1/4 of countries..." which hardly makes it any better, but at least the article can now be scrapped without beginning the sentence with a numeral.