I agree that teaching the mechanics of sexual intercourse to 4 year old children would be a mistake, but I'm guessing that "sex education" as it applies to this age group would cover things like "How are boys and girls different?" and "How to clean up properly after using the restroom" and "What to do if someone tries to touch you in a private place" and "How to ask for help if someone has been bothering you", and things of that nature. I doubt strongly that the children would be taught anything about actual sex, don't you?
We were all 4 and 5 year old children once. I don't know about all of you, but I have some very clear memories from that part of my life, and one of them is that when I went to kindergarten, every available space on the sidewalk and lower outside cement walls was covered with graffiti drawn by little boys. What did they draw over and over and over again? The little 5 year old boys drew crude cartoons of an enormous penis and two testicles. The little boys were not only proud of their little willies, they were obsessed with them, and took every opportunity they could to talk about them, draw them, joke about them, and show them off if they could without getting caught.
Do the gentlemen here who read this have memories of similar behavior from that age? You needn't be embarrassed, children are born without manners, it isn't their fault.

There was not a single male or female child in my kindergarten class who did not receive this peer-level "sex education", and we were all quite well acquainted with the construction and appearance of the male private parts, thanks to our little male playmates.
Seeing as how at age 5 penises and testicles were no big news to my crowd, I rather think I and my little peers could have benefitted from adult insights and explanations, instead of being introduced to the entire subject of sex, gender, and genitalia by other witless 5 year olds.
In elementary school, a little boy who was angry at me got all red-faced and shouted that he was going to kick me in the balls. A couple of his friends tapped him on the shoulder and whispered in his ear for a moment. After thinking with furrowed brow for a second or two, he came back with "I forgot, girls don't have balls. But I'll kick you in the tits!"
Ah, those precious, innocent school days!

If my little nemesis had taken a sex ed class designed for his age group, he might have known that "girls don't have balls", and spared himself some embarrassment.
They shouldn't have called it "Sex Education" because people imagine their children are precious and pure little snowflakes that shouldn't be told they have any private parts until they're 21. They spaz out. It should have been called "Intro to Health" or some such thing that wouldn't frighten parents who are terrified of sex.