Help! 4yo violent towards his 2yo sister

[SpoonieMum]

We're having a lot of trouble at the moment with our 4.5 yo son being violent towards his 2.5 yo sister. It's been going on a good few months and if anything is getting worse. I can understand that they'll fight and squabble, and if this violence happened in retaliation to a trigger such as pinching a toy for example, then I'd be less worried.
But he just randomly hurts her when she's minding her own business. This morning he shoved her head really hard into her dressing table causing her to bang it quite badly. Other times he'll just push and shove or hit her for no other reason than she's existing in the same room as him.
He is otherwise a 'nice kid' iykwim. He's obviously got some kind of anger issue but we're really struggling to fathom out what to do. Please help!