I've always known them to be aggressive by nature, but the truth is that they will often display aggressive behaviour when they're feeling stressed.
In their natural environment, Tiger Barbs are school fish and prefer to live in larger groups, so you're on the right track with having 4 together.
Your school may be just not big enough.
I've read that a school of 6 and more is what they prefer, but remember they will probably still display some mild snapping behaviour amongst themselves.
Check out the Wikipedia site: http://en.wikipedia.org/wiki/Tiger_barb
Read the "In Aquarium" section. It actually gives a good, brief, overview of their behaviour.
I personally feel that you are on the right track, matching barbs. Rosie barbs are a lot more chilled out, but even they, when stressed, will become aggressive.
I would try, if your tank size allows it, to try adding a few more Tiger barbs.
(Or replacing the Rosie Barbs with some bottom feeders.)
Oh yes- Barbs like to nip on long flowy fins. Avoid that.
Hope this helps!