I don't think year 4 is that early but I'm of the if-they-grow-up-knowing-it-it-won't-be-half-as-interesting exytraction (worked through my teen years anyhow! Mum was the opposite of prude- too much so probably)
Plus there are kids as young as year 4 starting periods etc these days so it all starts to come together now I think.
Intrestingly as this si the third person I've bumoed into (other 2 in RL) looking for advice on the same thing!
The NAS (Wales) had a discussion on this a while back, a Seminar- Temple Grandin iirc (too far from us to travel sadly). They may have some good advice: they certainly have literature on the subject (I agve my members lit list to another friend, but its there definitely).
I think you have, as a parent of an AS / ASD child to really emphasise the realationships aspect of all this, its all very well if someone says 'in a loving relationship' but you need to spend a LOT of time spelling out (social stories? carttons trips?) what IS a loving relationship; at what stage different things become appropriate (focus on things like maybe holding hands / kiss); the complex things like how to tell if someone fancies them; differeing sexualities; all the stuff that can get a kid into trouble if they don't understand it, but which doesn't come naturally to an AS kid.
Appropritate touching- now thats one a friend is having issues with atm!
I think it has to start young- my 7 year old atm likes erm, fiddling. Its non-sexual of course, but I find a slightly- absent minded @its only appropriate to touch yourself in your own bedroom, when there is nobody else present' is helpful: hopefully this information, clearly palced, will lead to other behaviours being practiced appropriately in a few years time.