Is there a homeschooling group in your area? You'll still have to search because there are a lot of reasons why people homeschool. Some groups wouldn't want the kids to have any info other than "God makes a baby for one man and one woman" and others homeschool so they can present a lot of info in just a different way. You can also check with the librarian in any major library or library network in your area.
I agree with the post about the abstinence only curricula being a massive problem. Usually the towns/states that push this have much higher rates of STDs and teen pregnancy. It's just not realistic, and it's totally devoid of the info that kids really need to make better decisions.
I think the points below about your kids getting the info from other kids on the playground are very valid. One advantage to them all being in the same class/program is that they get the same info. The worst thing for kids is to think their parents don't know anything, and to hear it all from friends. My mother gave me great info on menstruation, for example, before the 4th grade school program (which was just for girls - the boys got recess!). But she gave me NOTHING on sex and I was very clueless well into high school other than what I heard from friends or from suggestive pictures my brother and an older friend left on my pillow. I would have benefited greatly from a curriculum.
Maybe it would make the most sense for you to do something intermediate for your 4th grader first, to get him/her "up" to the level of the curriculum, then put him/her in the class program. Don't be afraid of it. Your children will thank you for it.