As a mom of three, Nonni of three and teacher of special needs for more than 3 decades, I think that you are right to worry. You obviously have a good idea of how to manage typical behavior, because your other two kids are doing fine. I'd strongly suggest that you have your little guy evaluated by a pediatric neuropsychologist. Whatever is going on (his neurology, stress from being 'the middle', other developmental concerns) he is negatively impacting your family, which must be hurting him on some level as well.

There are people out there who specialize in helping; reach out to them!

Good luck to all of you, and stay positive. He is only four, and there are ways to help.