I agree about MS Nucleus but for some reason it wasn't popular with my son. If I was homeschooling a young child again, I might have tried a combination of MS Nucleus and the TOPS series:
http://www.topscience.org/home.html (search the Elementary series). Also, the Let's Read and Find Out series eg:
http://www.amazon.com/Clouds-Lets-Read---Find-Out-Science-1/dp/0064452204/My son really loved The Magic School Bus cartoons and books at that age. He watched Curious George and Sid the Science Kid too. A science encyclopedia and various PBS/BBC documentaries were the bulk of his "curriculum" at 4. But it seems to have worked well.
There's a curriculum called Building Foundations of Scientific Understanding (BFSU for short) that is very good for ages 4-8. Available on Amazon.com and not at all dumbed down and very thorough for that age. But might be a little heavy for the parent/ teacher to prepare in advance.
