I'll preface this by saying I realise it could just be typical of this age, but I wouldn't know as it was a long time since I was nearly 4/actually 4 and DD is my first/only.
On a daily basis she comes home from preschool telling me her closest friends (2 of them, not in a group though, more both are 1:1 friends of hers) have said "you are not my best friend anymore" or "I don't want to play with you anymore". Mainly one of the girls in particular. My DD doesn't seem too fazed by this whereas I find it quite nasty.
Am I worrying about nothing? Is this just typical of kids the year before school (late 3s/early 4s)? Not a sign they will be bullies? 
Also how would you manage it? I'm not exactly sure how to respond to my DD.