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.
http://www.thedoctorstv.com/main/show_synopsis/281?sectio...
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.