Let me be candid here...your son is a human time bomb who desperately needs professional help. Get him to a competent child  psychiatrist ASAP.  Additionally, I would find a new therapist who specializes in working with kids who were sexually abused.  Any rape hotline or crises intervention center should be able to give you recommendations.  If cost is a factor, your county's social services should have the professional help you need with payment assessed on a sliding scale.  Frankly, your son sounds like he may need to be hospitalized until his mood is stabilized. 

     Your child's boundaries were violated and a lot of his rage may be tied to the fact that nothing was done to protect him.  Though I certainly understand what prompted your husband to overreact, beating a four year old until he has welts is child abuse.

Doesn't mean he doesn't love his son but his rage will not prevent your son from acting out. He is going to ape what he experiences.

     You have a hard road ahead of you but that if that boy is hurting himself, he is in a lot of pain.  Providing him the help he needs to deal with that is the first step on the road to recovery.