at 4 my son loved nothing more than putting on barbie heels and a princess dress. i told him there was nothing wrong with it, like his sister dresses up as a fireMAN. but you can tell him that if he wears it outside some people may say mean things and think he's being silly, but leave it up to him.
my son is now 4.5 and i'd say only 1 x every few months he'll do it.
dad was against it but i convinced him to keep his mouth shut.
very age appropriate.
and if you are worried he's going to be gay, no matter what you do he'll be gay anyway.
if you're worried he'll be a gentler soft boy, let him; he'll get all the girls
: )