yeah, as a few others have mentioned, i don't know how in the world a random stranger is going to know that the little 4-yr-old kid is a girl or a boy. and also, if you're afraid that some person is thinking disgusting things b/c of a topless child, then it really doesn't matter what type of kiddo, you probably are putting a shirt on the kid for some reason other than because it's your little girl (would you be okay with pervy thoughts about your son of the same age? i will venture a guess: no.)... therefore, you don't actually have to explain why girls wear shirts and boys don't, b/c that's not an issue right now, at 4 yrs old.
you've made it clear you're not going to change your mind on this issue, so i won't try to convince you of my point of view, that for your particular fear, it accomplishes nothing to put a shirt on your little kid. you are going to do it, and you feel that you should therefore explain yourself to the kid, which i think is a respectable thing (way better than "because i say so"!) so... if you are doing it to "protect" the kid, then maybe you should just tell the kid it's to protect their skin. it's not age-appropriate to talk about protecting from what -- to mention pedophilia to a 4-yr-old doesn't make any sense -- so if you're making it in age-appropriate language, you can just leave it at that it protects their skin. if you happen to think it's smart to protect them from UV rays and would rather not slather on the loads of chemicals in sunscreen, then you're really not lying very much even if you choose to say, specifically, it's protection from the sun.
geez, this really makes me wonder when and under what circumstances it's okay and appropriate to lie to your kids? in general, i favor honesty. but i really wouldn't want to burden a small child with concerns that are mine (it's my job to protect the kid, not theirs, as they're incapable anyhow), especially ones that could tweak out their psyche at too young of an age and give them a pessimistic view of the world. this thread has given me stuff to think about (and most of it tangential to the topic)! these forums are good at that, which is one of the reasons why i love them, even things that i start out feeling inclined to roll my eyes at.