If the arrangement is you take care of the kids and the dad doesn't see them maybe you could talk with the dad about rearranging the custody with the mom. That being said if the 4 year old has sensory issues & other ones . . I personally would take an interest in his differences and find a better coping skills for the child there is all sorts of things to try out there. He maybe just a little jealous of your son and honestly I'm assuming his behavior issues just need a little 1-1 and please remember it is not his fault he is not trying to make it a bad time for you .he is just expressing himself the best way he knows how. Give him a new outlet. There is some cool sensory things on lakeshore that the school systems use. I also agree you could benefit form having a good coparenting relationship with the mom being a united front and everyone being on the same page would only help the child in all areas of his life.

Your husband needs to talk to his kids and tell them age appropriately that they need to behave for you or else the kiddo will continue to give you a hard time. If you're on good terms with his mom maybe you could bring it up too? It's all about coparenting, and you're a part of your step children's lives too.