T.C.
It's very normal (and that's awesome out of your six you haven't had the issue...seriously!), so hopefully she doesn't feel too bad or like she's doing something wrong. Even the best, most attentive parents can have kids that hit, spit, scratch, scream, kick, bite, etc...
With hitting (or any negative behavior like that), at that age, they don't really understand what they are doing. We give it very little attention beyond a "People are not for hitting", then focus on the person that was hurt and give all the attention to them. We aren't mean or negative to the child who hit - they simply get no attention while we tend to the other child. Then, we take the child that was hitting, and have them practice being soft. We talk nicely and we say "You be soft like this" and have child pat softly. Then praise them for that.
So basically we give the hitting VERY little attention and then give lots of attention to the behavior that we want. If he gets a strong reaction from hitting, he's going to find he has power with the behavior. Give the behavior no power, and he's more likely to drop it.
Also, since he's so little, he's probably quite frustrated at not being able to verbalize what he's wanting. Then he's probably quite frustrated at being "misunderstood". So, I'd keep an eye open to stopping any situation that might lead to hitting. Sit nearby and grab his arm if he's about to hit, and say "be soft". Then ask him what he's needing (since he's hitting because he wants something), so a "Did you want to play with that toy and she said no? Here, come play with this" and try to redirect him or something like that.
And I know that's easier said than done. Baby #5 is due to arrive soon, and life is busy with so many little ones! I can see that it's not always realistic to sit and monitor, waiting for a hit. But IF you can, it might be helpful.
Anyway, hope that helps. I used to do what you say you are doing, and I never got anywhere. But I read this different approach from a book, and the results were MUCH better. I'd try really hard to approach it positively and focus on making the child feel good for doing the right behavior (even if I'm helping child to do the right behavior) and just ignore the bad since ANY attention gives it power and more chance it'll continue. Say "People are not for hitting", focus on the hurt child, then practice doing the right behavior with the child, and give THAT the attention...Good luck!