Bullying

I'm looking for some advice. Our 4yr old is in kindy at the moment, and for a while we've noticed that she observes who's there on drop off and then sometimes get clingy. I suspected something was up and asked her the names of her friends and if everyone is kind. She said everyone is kind, so I left it at that and always told me about the same friends so I left it at that. Then about 6wks ago a teacher mentioned that she often tells her she has no one to play with, despite her telling us that she plays with the same kids each day. I asked them to keep an eye out and let me know how she's going, then they mentioned yesterday that when it's the day that just her and her 'best friend' are there she's great. When the third girl is there, they leave her out of things and don't let her play with them. The teachers have said that they're making a big deal of her/encouraging her to play with other kids etc, but I'm really worried that she won't talk about it with us and just bottles it up. I've made it clear that I'm here to talk at any time and reinforce how loved she is. It's started to impact her confidence, she no longer wants to go to swimming and is now telling me she doesn't want to start dance next week. She's a sensitive soul with very big heart, and I don't want her to be affected by dealing with this at such a young age. Has anyone been through something similar? Any tips on the best way to approach this?