My DS is also aged 8 and in Yr 4 and is quite bright. He seems to be in the top sets for maths and literacy and goes to an extension group once a week with some of the other Yr 4 children. They don't have a G&T register at his school.
He is in a mixed ability class and he doesn't experience anything like your DD is going through. He seems to be popular and well liked and none of the other children seem to hold it against the children who go to this extension group.
As far as I'm aware, his teacher isn't in the habit of holding his work (or the work of any of the other more able children in the class) up as a "shining example" to the other children - thank God. I think she is more in the habit of holding up the work of any child as an example if she thinks they have done it well, or it represents a particular achievement for any given child - regardless of ability. Surely any good teacher should be doing this, shouldn't they?
In your shoes Katsh, I would be arranging to speak to the teacher (and possibly the head too) to get to the bottom of this. I'd certainly make sure they stop drawing attention to her in class over her work if she's finding that hard.
I'd think I'd also be looking at myself to see if there was anything that I was maybe inadvertently doing to exacerbate the situation. For example, have you maybe "bigged her up" a little too much? Does your DD know she is on this G&T register? If she does, maybe she has unwittigly let it slip to her peers in the past and maybe had a little boast (before the bullying started when she wouldn't have thought there was anything wrong in it) and maybe this has given rise to the problem?
Whatever, bullying is never acceptable and it has to be stamped on. I really hope you can sort it out soon. It sounds horrible what your poor DD is going through.
It makes you wonder if these G&T registers and labelling children in this way at such a young age is a good thing and serves any useful purpose when the likelihood is that most of them will level out at some point. Actually, I'm glad they don't have it at our school.