I had a very minor phase where Jake used to be aggressive towards Oz, they were similar ages to your children at the time (4 years and 8 months)
I found that giving Jake some one to one time when Oz was asleep helped a bit, as did telling him that the shoving about was teaching Oz to be aggressive, and that one day he'd be bigger and would shove back. It took a mix of things really, but we did get there in the end without any serious injuries, they are 9 and almost 13 now and both lovely boys.
Trogette - there's some interesting stuff there in what you said about "unconditional parenting" and I like the idea of demonstrating the right way to do something, rather than focussing on them not having done it.
I am curious though, I don't "punish" my children but I do give them "consequences" for certain things, which I try to make as relevant to whatever the issue was as possible.
Do you not give consequences either, and if not, how do you teach them about responsability for their actions? (age related obviously - sorry I'm not sure how old your children are

)