Hi T.:
I have a 4-year old son and he also has a double bed. We finally took off the guard rail a few months ago when we put his mattress into a captain's bed (drawers underneath). I was scared to death because the bed is so high off the ground. But he actually insisted that he was a "big boy" and didn't need it. He did fall out once (I made sure both sides of the bed are clear from all toys, furniture, etc. so that when he did fall, he did not get hurt. It sounds like your daughter is ready from your description. It's usually us Moms who are not ready :-)