If you are prepared to make some changes and dedicate yourself to a solid plan, take a look at Lynn Clark's book SOS Help for Parents. Follow its guidelines carefully and you'll see some nice improvement. See www.sosprograms.com.
i have the same problem though my daughter is now almost 10 years old. Although she doesn't hit other children as much, she does harass her kid brother and just refuses to listen. I have tried practically everything - from encouragement to time out but nothing seems to work. Please help!