N.B.
You could look at the Learn at Home series of workbooks/curriculum for homeschooling. I found them at amazon.com.
I have a friend who has six children, and she home-schools them all. As they have gotten older, she has gotten the oldest ones going to private school part-time, or else they would miss out on opportunities like orchestra and football. I don't know how she does it with all the kids home every day, but it works for them. It is possible. And four kids doesn't seem like that big of a family to me--I am the oldest of six, and my husband is one of eight!