HOw awful!! I dont think it is appropriate that your daughter had to go and tell the teacher what happened - I know I would have been v embarrassed to say something like that to a teacher (let alone a male one) and she has done nothing wrong so why is it put in her court to discuss in depth with the teacher. she told you so you can help her. when i was in high school someone flashed at me pretty aggressively on a bus on the way to school (and a friend). we told the teacher and then had to describe in depth to police and our school principal. i was SO embarrassed and justwanted to drop it... woudl have been a lot more comf if my mum could have handled it for me (and iw as a lot older than grade 4)
i think at this age all parents need to be involved, it isnt even that the boys are 'bad' - they are just learning, mimicking bad behaviours and there is an opportunity to stop it now before they are older and more impactful.
let us know how you get along.