I have heard of middle school down as far as fifth grade and up as far as ninth (not usually at the same time), but not as far down as fourth. Can't imagine doing middle school at nine. Where I live it's now K-5, 6-8, 9-12, which makes the most sense to me. I had to do K-4, 5-8, 9-12, which I didn't like at all (middle school was by far the worst part of my education).