I have talked to his parents about this and they are as stumped about what to do as I am. Do you have any advice for us? He is a very young four-year-old and an only child.
Try to work with the parents to set up a positive discipline system so that they can follow up at home. Divide the day into segments and allow him to earn a sticker or a check mark for making it through center time or outdoor time without hitting someone. Let his parents know each day how he has done. When he has earned three (or whatever number you decide) stickers he can have an extra bedtime story or help you make a cake.
Let him know, too, when he is being good in your class. You say that at times you just want to hug him -- do it. Give him a hug when he is having a great day and tell him why he is getting it. Let him be your helper and pass out cookies when he has not hit anyone.
The way that you describe him becoming "enraged" at the other children sounds like it may take more than these suggestions to make a difference. If your school/preschool has a school counselor, ask that person to help this boy. You may want to encourage the parents to talk with their pediatrician about referring them to someone in your community who provides therapy for young children.