We began teaching our ds and dd about modesty at ages 5 and 4, respectively. It didn’t come easily at first because they had been used to changing clothes in front of each other, bathing together until ages 4 and 3, and what made it even more difficult was that they shared a room. We shared that God wants us to be modest because our bodies aren’t meant to be shown to others. We’ve never dealt with the specific question of “Why do girls have to wear shirts when boys don’t?”, so I’m not sure how to answer that. Our ds does wear a shirt all the time, even at the swimming pool, so his sister never sees him “topless”. ha! In our case, modesty between siblings wasn’t something we really enforced until about a year ago, and I began to realize that if I didn’t enforce it early enough, they might eventually wonder why one day they were allowed to take a bath together and the next day they weren’t.
Two-piece bathing suits are something else we don’t allow. I know little girls are adorable in bikinis, especially when their pudgy tummies poke out. But, I didn’t want my dd growing up wearing bikinis. Someday that tummy wouldn’t stick out anymore and there would be breasts and hips where there aren’t any now. I didn’t want her to be allowed to wear a bikini until she matured, when I would have to say, “Ok, your body is too mature now to wear a two-piece. We’ll have to get you a one-piece this year.” I just know her response would be, “Why, Mom? You’ve always let me wear a bikini. Why do I have to stop now?”
To avoid that conversation, we’ve just never allowed two-piece bathing suits. With your daughter, you could see if that approach would work for you. “Sally, I know you like to run around with your shirt off, but that’s not appropriate in our home. We have to keep our bodies covered and be modest.” That way, when she’s 12, she’s not saying, “But Mom, you let me run around without my shirt when I was 4 (and 5 and 6 and so on), why can’t I now?”
To me, modesty might be something a young child doesn’t understand right now, but we do have rules in our home; and those rules will be followed. Understanding might have to come later. I now have a very modest 5yo dd and a very modest 6yo ds. They didn’t understand what modesty meant at ages 3 and 4, but they do now, and they don’t question it.
I’m also sure that she will develop socially to understand that Mom and sisters or girl cousins don’t go topless, so she won’t want to forever either. Like Gem said, she won’t be 4 forever! Hope that’s helpful.