I took dd1 to a 4th birthday party at the weekend. Dd1 is nearly 4, very sociable and in general I trust her around other kids. She was shoved and pushed over repeatedly by the same two boys (she knew one but not the other). I ended up standing over her to intervene every time it happened. I haven't had to do this since she was a toddler (when all kids need close supervision) but the parents of the mean kids didn't bother even though they saw dd1 crying more than once.
I font expect 4 year olds to behave perfectly or uniformly but aibu to expect parents to supervise their kids when thru have already seen them push and shove and be very mean.
The only comment made was 'oh is dd1 ok? There are a lot of boys here.....' Theta feeble too - both dd1's best friends are boys and do not behave like that. Being a boy doesn't not negate bad behaviour .