I think 4 is a very good age for this actually. Its been proven that children who have been taught to correctly name their body parts and to understand the differences between adult/child relationships are less likely to become long term abuse victims because they know how to verbalise what is happening to them.
Maybe it shouldn't be up to the schools but the fact is there are thousands of kids out there who's parents are not open with them and are horribly embarrassed at using real terminology when taking to their kids. Children need to be educated as young as possible, thats the world we live in now a days whether we like it or not.
I'd hardly call those lessons sex ed anyway but generally speaking real sex education lessons need to happen earlier and be more in depth than they are now. There were girls in my class becoming mothers before we ever had a sex ed class, that's unacceptable. Whats worse is that it hasn't changed much in the 11 years its been since i left secondary school. The system is failing our kids and instead of parents getting on their high horses sounding off that their kids are too young they need to wake up and see whats happening around them. We have one of the highest teen pregnancy rates in Europe and its directly linked to our appalling sex ed lessons!
Ok i'll stop ranting now