I would only be repeating everything Annie has said here...this in my opinion is how to help your stepson ..good luck
It's too long in duration, and too far from the problem. Hitting, kicking, screaming and pinching at preschool often come from things before going to school, such as things the day before, like not enough attention, too much overdramatic T.V., not enough sleep, not enough exercise, not enough feeling of security at home. (A child who is split between two households can find it hard to stay secure.) In other words, it's not so much a school problem as it is that a kid takes a "load" into school and lets it out. One can't just punish and expect that to have much effect on the acting-out behavior. It's a result of other things, not something that just arises out of the blue in the school.
You and your husband should have a big discussion about what the child is reacting to. It might not be on the list above, or it might, or it might be a combination of those things. Whatever it is, you can begin to work on the specific problem. Also, keep in mind that stopping a kid from hitting or acting up in preschool is just a multi-level approach, and you need all the levels.
Kids in this day & age don't get enough exercise; it's really easy to plunk them down in front of the TV to get things done, the images on even kids' cartoons are pretty rough in some cases. My son reacts really well to calm days at home with the TV off, when I can manage it (i.e., when I don't have to rush to get the house together for visitors who are coming, or things like that). Even when the TV is on, I'm monitoring it. If I hear anything with testosterone-laced characters doing martial arts or loud, suspenseful music, that's it, we're off to Max & Ruby. But I also try for at least three peaceful days at home out of five (by peaceful, meaning no television at all). It cheers him up because of what he does instead. When he's not just mindlessly watching TV, he's doing art, or playing with a ball, or building with toys, or doing other things that are more fulfilling than TV.
Talking to him in advance and practicing options helps -- what can he do when he gets mad at school? take calming breaths, talk to the kid, tell the teacher the other kid called him a name, etc. etc. (Go over the list a lot when he's not at school, and practice the coping mechanisms.)
I also used both rewards (stars on a calendar that added up to being able to get a toy) and consequences (not getting a treat after school that he knows is in the car) when I was trying to keep my son from acting up in preschool. As you drive him to school, show him an upcoming reward he will get if he has a good day (a favorite snack), and if he's had a bad day, don't give it to him. Encourage him with a gold star on the good day, remove a star when he has had a terrible day, things like that.
Basically, your husband and yourself should put some thought into it and work it on all levels. It sounds like your husband has only one approach, and that is himself getting mad or frustrated and forcing the child to sit on the bed. He should be gently guided into trying to put himself into his son's head a little more. A 4-year-old is probably not trying to be defiant or bad, he's just reacting to things out of his control or his ability (with his young emotional system) to deal with. A kid who is feeling good and serene when he walks into preschool will not lash out on a regular basis in the preschool.
Good luck!