Sadly IME the dynamics definitely changes when they hit this yr4 age group, I can only describe it as more sinister, relentless & manipulative than anything previous, I was quite shocked it kicked in so young, I expected yr6/7 as it was in my day, but the Head had picked up on the possibility of it before I had even realised & was concerned about DD missing a chunk of School due to health problems & how that would affect the dynamics - I expected the usual spits & spats, DDs queen bee friend had always been a problem, seen the same crying wolf behaviour from her in the earlier years too, but this time round it was much different, relentless & controlling the other DCs to do her bidding, or they risked being at the brunt of it too
Basically my DD also had problems since much younger, same core group of friends since the beginning of School, similar things happening at 6 to your DD Jooly, but having had the whole group here a few times I had already noticed things changing, it was so sudden too, 1 minute it was the same sort of thing you describe, not good, but sort of little girls being little girls & then suddenly it was like young women, much cleverer, more subtle & more difficult for teaching staff to see through it as it was gang mentality - I spotted another of the group being picked on at a party here -
I watched QB orchestrate it, yet she never at any point took part, instead had another girl literally hound another of DDs friends every time she went near the food table, subtle jibes, "are you really hungry again" what, you at the food again" "how many plates have you had " etc etc - the word fat was never mentioned, but the implication was clear, but nothing I could pull her up & tell her off so to speak, DDs friend still ended up in tears though, because there was no doubt of what was meant 
- I was gobsmacked at how adult it had suddenly become
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DDs situation was similar too, chip, chipping away, but when she went off & left them they hounded her - QB having side kick telling her "your either our friend or you might as well go kill yourself" - Young Male teacher getting the group together to see what was going on, had my DD speaking out against the whole group on her own against the rest of them, none of the others dare speak up, so like your DD, mine was seen as the source of the problems because in her frustration & upset she was naturally getting stroppy when sat with all her friends, lying & blaming her - took a couple of weeks to get that put right thankfully the other girl was eventually brave enough to speak out, when spoken to alone - some of the others owned up to jealousy of DD too
I've gone through phases of wanting different friends for DD, but its just not that easy, ironically out of all of DDs friends she probably has most in common with QB in terms of interests & ability, difference is DD thinks its great, QB sees DD as a threat, & lest face it, it IS what she's going to face in life - she's very pretty & very clever too, add to that very popular with the whole School & she's a target for this sort of thing, I'm actually happier to see her dealing with it head on, than running off & finding new friends, but as I said I also had the support of QBs Mum over it, she talked to her DD at length about whats acceptable & not - I know she will always be a problem, so does DD, but she's no longer controlling the group, nor able to manipulate staff etc & she knows she's met her match in my DD, which I think is actually better :)
The one thing I realised I forgot to add last night, a good bit of advice I got on here was to try & find your DD some sort of out of School activity away from this group, doing this really did help my DD a LOT whilst going through the worst of it :)