it depends on what spot you are vs the 9/11 dual. attack whichever base you can get your cats to quicker. It really just comes down to how well you manage your resources, and your micro with grunts. Conserving your grunts so they can kill what's trying to kill your cat, is what wins it.
would you say its better to counter dual from 4?
no because 4/5 is super easy to dk. And when you counter dual even if you're 100% efficient at it vs a good player you will still be behind. When i'm 4 i chop out and try to cat. I bring 2 peons to their base, build a cat at home, rep a rax outside their base, put up 2 towers, and a 2nd rax. That way you can continuously pump *level 2* cats, and grunts, and tower them. If you break in one base, you win the game basically. If you break in their base, and still lose, you need to improve on your APM and res management, with multi-tasking.
As far as the strategy " counter-dual " goes, doing the same thing as they are, AFTER they do it, is insanity. It's all about being one step ahead, not a step behind.