Good post...and very interesting.
Abuse is the indication of something happening more than once, or on a regular basis. I would say that playing doctor is pretty much something every kid does...they're curious. Boys are curious as to why girls don't have the same "equipment," and girls are wondering why they don't have one of THOSE!...
Perfectly natural, IMO, for kids to lock themselves into a room and say, "I'll show you mine, if you show me yours." Pretty much all of us have been curious at one point in our lives, and many of us have "seen" the parts that the opposite gender has.
However, these are kids - so it is innocent. 4-5 year olds do not even hold the word "abuse" in their vocabularies. This is not to say that a child that young is incapable of abuse, because they ARE - but they are unable to define the word. Say Child A loves to punch Child B in the stomach every day...this would classify as physical abuse, bullying, etc...but Child A might interpret it as "I'm only playing with him, I hit him because it's funny."
Back to the original topic...There is a bigger problem at hand when one or both of these kids KNOW that showing off of their private areas is wrong and/or inappropriate.
What I'd like to know if this was going on over more than one or two or three occurances is - where were the parents? How come kids this young are left unsupervised long enough for them to get this far "into" the game of doctor? I believe a good parent will always know what their child is doing - even if there is a friend over and they send the kids "off to go play." In the rare case that a parent will catch the kids in action, then a stop should be put to that kind of play, and an explanation rendered to the children as to why this is wrong.
Hopefully that'll take care of the issue. If not, then there is a problem.