"However, what I think we can all agree on is that a four year old does not take it upon herself to lick a boy's penis. "
i'm with chippingin here, sakura. i really do not agree that a four year old girl wouldn't like a boy's penis without something else going on in the background.
honestly and truly, in the last few months my 4 year old dd and her pals have ALL been licking each other, god knows why, but they have. girls and boys, they come up behind each other and lick necks, faces, hands etc. they do it to us too. it's just a phase they are all going through, for some weird reason.
i do think the bigger question is 'what was the winkie doing out in the first place, ds?' but the OP has provided NO further information.
however i genuinely do not think it is that unlikely that dd or her male and female friends would end off 'doing a lick' (which is what they call it) to a winkie that was out and about, i really don't. it's a funny dare thing they have going at the moment, i wouldn't rule anything out. (apart from sexual abuse).
funnily enough, imo this is related to that other thread insofar as this is an age when children are beginning to learn about boundaries, about private places etc, and it will depend on how their parents are speaking to them, or indeed whether they have yet, whether this would be a taboo action or not. so a four year old girl may not have had that conversation with her parents, while a five year old boy at school may have done. and vice versa. impossible to knwo without more information.
but if you are really worried, worriedmum, i would definitely speak to your friend first. i wouldn't be horrified if dd had done this to someone else (wouldn't even be particularly surprised tbh) but it would be the springboard to an important chat about appropriate behaviour etc.