"i think these things teach children resilience."
I know it's been said but it doesn't teach resilience, it teaches little boys that they can be boisterous and do what they want, regardless of who it upsets.
"It teaches a little GIRL that it's OK for a BOY to do as he pleases, inc being unkind to her for no other reason that he can."
Absolutely. In every social experiment when they put cameras in play areas, it is largely the boys putting a stop to the girls games, without being admonished for it. The girls are told to calm down and stop upsetting others, but the boys are indulged. There's still idiots about that tell girls that they should put up with being picked on because it means the boy likes her.
There has never been a test done, including in schools were the boys don't get away with it, because it's seen as being in a male's nature to behave like that.
Watch a 'rough' family with mixed gender children, you'll find that the boys even pick on their Sisters without much being done. Whereas the girls are told to stop whining and that "he didn't mean it".
The Adults were out of order for not telling him off, even if he was a big 4 year old and not as old as the OP thought.
Watching the children choose Halloween costumes in the shops, i'm sick of still seeing girls being forced to wear dresses,. One little girl, that I saw, was trying to tell her Mum that it meant she wouldn't have enough fun on the bouncy castle (she wanted to be a skeleton). She was shut down, because that "wasn't a girls costume" and "ha ha" 'd at by him.