What to do if your girlfriend is really good friends with their ex?

How would you feel/ what would you do if a girl you were interested in was still "really good" friends with an ex where they dated for over 4 years? I know you shouldn't tell her who to have a friendship with but to me really good friends is different than being a friend/friendly to the person and I feel I have a right to protect myself. Also we're both Christians who don't believe in premarital "benefits".