I enjoy the show "The Doctors" but can't always catch it because of the time slot. It's on at 5:00 pm and it's a talk show panel of Doctors who answer ordinary, thought-provoking questions. I happened to watched it today and this very same question was asked. Scroll through the synopsis of the day and you'll come to the question/answer. It's entitled "Nudity in front of your children." There was more said on the show, so this is just the gist.
Generally, they suggested that "If YOU are questioning whether they are too old, then they probably are" and that's about at age 2 - 3 years old. Or if the child is looking up and saying "Gee, Daddy, what's that??", then it's time to reconsider the shared-shower arrangement. For us, no more showers or baths, together, probably from 2 1/2 years and older.
So... for my vote, I believe that 4 1/2 years old is too old. I think children, at that age are way too inquisitive and, who knows what she could, innocently, mention to others (friends/adults/teachers) in school and what could be misinterpreted.