next to her he seems like a nightmare but next to other boys his age he's well behaved
He's not though if he's kicking and biting.
Is he a monster?
No he's not a monster either. He's a 4yo who is learning.
I would go into it with the teacher in a "we're all in this together" mode and work out a plan of action. She's saying they're all doing it, and you want to know how it's being discouraged. You want to know how they're dealing with it, and what you can do to support the school in helping your ds (you can't speak about the other children) to develop control not to behave like this.
Don't go in assuming it's all them, and your ds being led. If the teacher says that, fine, and you work on it from that angle. But it doesn't sound like that's what the teacher thinks, so you need to deal with it from that side.
Also remember that "Violent stories" from a 4yo can depend on the teller. Dd1 you used to hear how A had helped the teacher, B had looked after C when they'd fallen over, D had got a sticker for saying thank you...
Dd2 I'd hear how Z had pushed the Lego tower over on purpose, Y had hit X and W had thrown sand in V's face...*
Yes, dd2's class was more "interesting" but not to the extent that you would have got the impression by listening to them.
They're still like that at secondary. Dd1 will be telling me how one person had got an award from XYZ while Dd2 is telling me how M and N aren't talking.
*Ds only told me the score at the lunchtime football and what he'd eaten for lunch 