DS (3.9) did not cope well with the arrival of baby DD. People said it would get easier after the first year and it did, briefly, but he seems to have regressed and keeps bumping and knocking into her. It's not just DD - he has an 18 mth old cousin who he has a habit of knocking over. 
I'm not sure if there's something going on here as he has been uncharacteristically aggressive at nursery too. I can't think of anything that has changed to unsettle him recently.
In terms of what I'm doing, I try to encourage positive interaction and if that isn't working, I try to stay calm and give a warning (if the first incident isn't a major infraction) and then give time out. Sometimes I end up shouting because I'm so fed up. I've seen DD jump when he comes into the room which breaks my heart. It's so hard. I love him, but I really don't like him when he's like this.