this is around the age we started these conversations, actually.
it's the age when they really start making friends and exploring the "outside" world more.
when i first started seeing my child being bossed around (bullied) by an older neighborhood kid, we started talking about how NO ONE gets to be mean - it's not just him that has to be good. if there are other kids that are being mean, we don't want to be around them. when they can be nice, then we will play with them. i said these things in front of the child too. i wanted all involved to know that my son deserved better. it worked at the time. the older child knew if i was around she had better behave. although, she was like 6-7 to my son's 4, at the time.
i think it's always good to tell them how to handle difficulties. at this age it is a lot of, "if sally isn't going to be nice, you don't have to play with her. no one gets to be mean to you. you're too special for that. you go play with bobby until sally is ready to play nice."
kids' friendships are fluid a lot of times especially at this age. the kid who was "mean" last week will be their bff, then you'll go back to hearing "i don't like her she's mean." they may not be bad kids (although from your question it sounds like this little one has a history of less-than-great behavior) but they might just disagree with how to play tag.
one time my son came inside crying, all upset and saying all the kids outside were mean and he hated them and he was never playing with them again (or something similarly over-dramatic). when i started asking questions, i found out that little jimmy wanted to play power rangers while my son wanted to play transformers, and this was the result of their disagreement. wow. earth-shattering stuff. but you'd have thought they had held him down and beat him, as wounded as he was acting.
99% of the time i tell the kids to work it out themselves. as long as someone isn't being physically hurt, i do believe it's best for them to learn to get along on their own. but i do impress on my son (along with good behavior for himself) that others should be good too, and if they're not - he doesn't need to be around them.
ETA- ok looked at your first question (and answered). i think that some moms might be a little over-sensitive and really seem to be TRYING to find fault with YOU in that scenario. don't sweat it. you're not a bad person. you didn't need to apologize. a couple of those answers were unnecessarily snarky. i would wonder the same things if i was in your shoes.
but it will all work out. your daughter will be fine.