4 year old/first born child behavior issues

Hello,
My son who turned 4 in January, and is our first born child, is extremely rough and aggressive with his 2 year old boy/girl twin siblings.  He is especially rough with his brother.  Whether its pushing them off the couch, kicking them in the bathtub or squeezing them as tight as he can, he just cant seem to keep his hands off them.  Its fairly obvious that he does this for attention since he acts out most often when my wife or I are in the room. He will even smile while getting disciplined.  Time outs are about worthless at this point.  The one leverage we did have was taking away TV privilidges.  He has since figured out that one of us will end up giving him more attention to keep him entertained.  The kids do go to day care/school part time and he shows none of these behaviors to other kids.  In fact, a few weeks back when our boy twin was having a rough day due to his sister being home sick (the twins are quite attached), They brought him to see our 4 year old.   He took his little brother by the hand and walked him around the classroom showing him to all his friends.  Once they get home however, its a whole different story.  How can I get him to stop this aggressive behavior towards his siblings?