I know that this wasn't really the question...but does he have a legal right to see her? I wholeheartedly believe that children should see both parents, but sometimes one stable parent is in the best interest of the child.
My daughter is the same age as yours. Her dad has seen her less than 6 times in the last year. We live within 30 minutes of each other. At least half of those occasions involve me taking her to him.
We have a legal visitation agreement. Even though he has NEVER shown up on his designated day or time, I have not restricted access.
All that being said, if you haven't already, meet him in person to establish the parameters of the meeting. If you feel like he is sincere and willing to make a consistent effort, then I believe you should tell her right away. She is going to be confused enough without y'all intentionally adding to it.
On a side note, I always know when my ex husband has a new girlfriend...he starts coming around to see her and wants to take her for the night. He likes to show her off...he did the same thing when he was dating me. If only I had been able to see the writing on the wall!
Also, I would urge you to get a legal visitation and custody documented if you haven't already.
Good luck. I can honestly say that I know how you feel.