My opinion would be, as you have in your mind already, she is too young to know what her intelligence level is accurately, too young to diagnose ADHD, too young to medicate. If indeed she's behaving like normally developing age-mates, I don't see what the problem is. That doesn't mean she doesn't have a level of intelligence similar to her sisters. Maybe she's a later bloomer and will turn out to have an even higher intelligence level. You may not know for a few more years.
Enjoy your interesting girls
and be patient. It's great that you're keeping a watchful eye. Also keep in mind that there are plenty of other issues that have similar-appearing symptoms to ADHD (see e.g., the Eides' The Mislabeled Child for discussion)
