HELP!!!

I have a 4yr old son..who we adopted at age 3.5 years. We met him at age 3.5 and he moved in with after two months of the first meeting and being chosen as the adoptive parents for him. He has really only lived with us for a little over a year.  He had never been in a preschool setting until he came to live with us.  Since my husband and I both work, we definitely needed child care for him.  New home, new pets, new parents, new environment, new school...etc...Needless to say, he has had and is still having adjustment issues.  He took to us as if we were his natural parents and we fell in love with him at first sight.  There is no question that his social skills with his peer group need work.  Maybe this is all children at this age...I don't know...it is so frustrating because he is such a great kid...everyone in the church family loves him and he wins over everyone's heart...but when he gets upset..which seems only to be in the preschool environment and sometimes in Sunday school...he reacts faster than he can think...I realize he is only 4--and the ps teacher has already talked about maybe having him go somewhere else...Mondays are his worst!  He acts out...doesn't want to listen...does not keep his hands to himself...hits...sometimes fights time out in class...I dread Mondays!! I know that I'll probably get a call...it is so strange...he has a great weekend with us(he is the only child with us) and  does awesome at home.  He can tell you why he should not hit and what the rules are, why he should listen, etc.  But he just reacts and lashes out..with kids and some of the teachers. I know that the preschool's are limited in what they can do..but it is impossible for me to constantly go down to the school when this happens which is basically every week... We have tried time outs at home based on his school behaviour, spanks, talking with him letting him tell us what has happened and what he should do to avoid the lashing out.  He is aware that the school may kick him out.  We started a sticker chart at home which reflects his behavior at school...whenever he wants to behave he will...but who knows what goes through his little mind when he is on a "terrible" roll at school.  He just becomes defiant with the teachers...even putting him in the office nowadays does not seem to assist the issue.  Help!!!  WE love him alot and are very confused how to discipline this type of behavior since we are not there with him all day...and even if we were...he would be an angel with us...because we have  set very apparent boundries for him.  Please let me know how we can help him through this...don't want him to get kicked out....and he'll be starting kindergarten in August....and I am afraid that this behavior will  continue into that new year/school.       So..how do we effectively work on his behavior away from home( school) at home...???Thanks.