Let her know you have heard reports she is talking behind your back, but be nonconfrontational. Be specific about what you have heard that she has been saying and ask why. And before you approach the topic with her, decide whether your goal is to keep the friendship and make it stronger, or to confront her and tell her off and write her out of your life. Also (probably more importantly), figure out whether you would be okay if YOU are the one the rest of the group decides to drop, rather than her. Keep in mind that if within your friend group you all tend to talk about other people without their knowledge, then it is quite likely that at least several of the 4 are also doing the same about the others in your immediate group.