You were wise to not overreact to her play with the other girl; experimental play with kissing is typical at this age. You do need to let your daughter know that play like that is not appropriate with a boy or a girl and that she should not do it any more. Try to be aware of what she is watching on television as well.
You don't indicate how long you've noticed these behaviors. If they continue over time, you may want to talk with your pediatrician. Whatever direction your daughter takes, make sure that she knows that you accept and support her.