I understand the concern and it is important to teach the values of inclusion and politeness. At the same time, perhaps the kids she doesn't want to play with aren't that nice or are doing something that makes her not want to play with them. As an adult, you may have to get along politely with co-workers you don't like, you may even have to work with them occasionally, but you certainly never have to engage in play with them. Is just wishing to play on her own or is it that she has a couple of friends she likes to play with and a few she doesn't?
Has anyone asked her why she doesn't want to play with the other kids?
Has anyone role-played with her ways she can be inclusive keeping her concerns in mind?
Has anyone spoken with her about the concept of inclusion and why it is so mean to just flat out say, "I don't want to play with you?"
Is she allowed some space to have independent and individual play? She may be part of the 25% of the population for whom "alone time" is how batteries are recharged and too much stimulous is sorted through and brought back down to a calmer level. She may be wishing to just play calmly and alone because she needs a break. School can be pretty exhausting.
Outside of these things, if she is refusing to play just because she can and it hurts the feelings of the others, then working on figuring out why she would want to be mean would be a good way to go. This type of emotional bullying is often indicative of self esteem issues, control issues, and a host of other possible problems. Most of the time though, 4 year olds simply aren't that snobby.
I don't think this is likely, because you say she does this to her sisters too, but She may need a little guidance it is around 4 or 5 they start noticing differences like skin color. She may simply not understand something about something she has noticed that you or I wouldn't ever even consider. If she doesn't want to play with some one because of soemthing physical, she may need some lessons in differences and how action is more important than looks.
If she has a legitimate reason for not wanting to play with these other kids (which is possible), she should be repsected and given some options and ploys she can use to help herself politely out of tough situations. Perhaps she feels the others are really bossy (older sisters are known for being bossy). Maybe they say mean things or make her feel bad in some way. . . Instead of assuming it is unacceptable behavior and she needs to be taught to get along, perhaps being taught how to get along requires a subtler lesson than you realize. Please ask her for her reasons when you are not upset and you are ready to listen, paraphrase and summarize - if you haven't done so already.